Data transmission method, access network device, terminal, and communications system

ABSTRACT

This application provides a data transmission method. In the method, a first access network device and a second access network device establish a data radio bearer (DRB)-based tunnel and a session-based tunnel. The second access network device sends a Packet Data Convergence Protocol (PDCP) layer data packet to the first access network device via the DRB-based tunnel; and the second access network device sends a Service Data Adaptation Protocol (SDAP) layer data packet to the first access network device via the session-based tunnel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/581,151, filed on Sep. 24, 2019, which is a continuation ofInternational Patent Application No. PCT/CN2018/080396, filed on Mar.24, 2018, which claims priority to Chinese Patent Application No.201710183306.9, filed on Mar. 24, 2017. All of aforementioned patentapplications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This application relates to the field of wireless communications, and inparticular, to a data transmission method, an access network device, aterminal, and a communications system.

BACKGROUND

With the continuous increase in user requirements, more in-depthresearch is made on a new radio access network (New RAN). The new radioaccess network can provide a shorter delay and higher bandwidth andsupport a larger quantity of connections than an existing wirelesscommunications system, to meet growing mobile communicationrequirements.

In the new radio access network, quality of service (QoS) management isbased on a flow and includes: establishing a protocol data unit (PDU)session between an access network device and a core network, where thePDU session may include a plurality of flows, and different flows mayhave different QoS requirements; providing, by the core network, a QoSrequirement of a flow for the base station; and completing, by the basestation, mapping from the flow to a data radio bearer (DRB) based on theQoS requirement. For example, the base station maps flows having a sameQoS requirement to a same DRB and transmits the flows on the DRB.

During data transmission between base stations, for example, in ahandover or dual connectivity (DC) scenario, a tunnel is establishedbetween a source base station and a target base station, a terminal datapacket on the source base station is sent to the target base station,and then the target base station communicates with a terminal. Becausethe target base station and the source base station may configuredifferent mapping relationships between a flow and a DRB, the terminaldata packet may be lost or repeatedly transmitted or the like when beingsent to the target base station or being subsequently transmitted overan air interface. As a result, communication continuity of the terminalis affected.

SUMMARY

Embodiments of this application provide a data transmission method, anaccess network device, a terminal, and a communications system.

According to a first aspect, an embodiment of this application providesa data transmission method, including:

receiving, by a first access network device, forwarded data packets froma second access network device; mapping, by the first access networkdevice to a first data radio bearer (DRB), a data packet in theforwarded data packets that includes no flow identifier, where the firstDRB is corresponding to a DRB of the second access network device; andmapping, by the first access network device to a second DRB based on asecond mapping relationship, a data packet in the forwarded data packetsthat includes a flow identifier, where the second mapping relationshipis a mapping relationship between a flow and a DRB in the first accessnetwork device.

The first DRB and the DRB of the second access network device each meeta first mapping relationship, and the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device. The first DRB is a reflect DRB of the DRB of the secondaccess network device.

The forwarded data packets include at least one type of the followingdata packets: a Packet Data Convergence Protocol (PDCP) layer datapacket that is of the second access network device, to which a sequencenumber is allocated, and for which no reception acknowledgement isobtained from a terminal; a PDCP layer data packet that is of the secondaccess network device and to which no sequence number is allocated; anda Service Data Adaptation Protocol (SDAP) layer data packet of thesecond access network device.

On a premise that each access network device can independently set amapping relationship between a flow and a DRB, the data packet that hasa flow identifier is mapped to the second DRB for transmission, and thedata packet that has no flow identifier is mapped to the first DRB fortransmission. Therefore, a data packet transmission manner can beselected based on an actual network status, to avoid the followingproblem: In a handover or dual connectivity or another scenario, a datapacket is lost or is repeatedly transmitted because each access networkdevice independently configures a mapping relationship between a flowand a DRB. This improves terminal service continuity and improvescommunication quality.

In a possible implementation, the first access network device sends, tothe terminal on the first DRB, a data packet in the forwarded datapackets that includes no flow identifier and to which a sequence numberis allocated; and the first access network device sends, to the terminalon the second DRB, a data packet in the forwarded data packets thatincludes a flow identifier and to which no sequence number is allocated.The data packet may be a PDCP layer data packet, and therefore thesequence number is a PDCP layer sequence number.

In a possible implementation, the first access network device sends, tothe terminal on the first DRB, a data packet in the forwarded datapackets that includes no flow identifier; and the first access networkdevice sends, to the terminal on the second DRB, a data packet in theforwarded data packets that includes a flow identifier. The data packetthat includes a flow identifier may be a PDCP layer data packet to whicha flow identifier is allocated, and/or an SDAP layer data packet.

In a possible implementation, the method further includes: routing, bythe first access network device to the second DRB via an SDAP entity,the data packet in the forwarded data packets that includes a flowidentifier.

In a possible implementation, the receiving, by a first access networkdevice, forwarded data packets from a second access network deviceincludes: receiving, by the first access network device, the forwardeddata packets from the second access network device via a tunnelestablished based on a DRB and a tunnel established based on a session,where the tunnel established based on a DRB is used to transmit a PDCPlayer data packet that is of the second access network device and towhich a sequence number is allocated; and the tunnel established basedon a session is used to transmit the SDAP layer data packet of thesecond access network device, and/or is used to transmit a PDCP layerdata packet that is of the second access network device, that includes aflow identifier, and to which no sequence number is allocated.

In a possible implementation, the receiving, by a first access networkdevice, forwarded data packets from a second access network deviceincludes: receiving, by the first access network device, the forwardeddata packets from the second access network device via a tunnelestablished based on a DRB. There may be one or more tunnels establishedbased on a DRB. For example, when there are two tunnels establishedbased on a DRB, one tunnel may be used to transmit the data packet thathas a flow identifier, and the other tunnel may be used to transmit thedata packet that has no flow identifier.

In a possible implementation, the receiving, by a first access networkdevice, forwarded data packets from a second access network deviceincludes: receiving, by the first access network device from the secondaccess network device via a tunnel established based on a session, thedata packet in the forwarded data packets that includes a flowidentifier. Optionally, the first access network device routes, to thefirst DRB via the SDAP entity, a PDCP layer data packet that is receivedfrom the tunnel established based on a session and to which a sequencenumber is allocated, and routes, to the second DRB, a PDCP layer datapacket to which no sequence number is allocated or the SDAP layer datapacket.

The foregoing various tunnel establishment manners are applicable to aplurality of forwarded data packet transmission scenarios. A forwardeddata packet that includes a flow identifier, a forwarded data packetthat includes no flow identifier, a forwarded data packet to which asequence number is allocated, and a forwarded data packet to which nosequence number is allocated all can be transmitted via suitabletunnels. Therefore, a packet loss or repeated packet transmission in adata forwarding process is avoided, and network performance is improved.

In a possible implementation, after sending of the forwarded data packetmapped to the first DRB is completed, the method further includes:releasing, by the first access network device, the first DRB, to save asystem resource.

In a possible implementation, the first access network device may sendthe second mapping relationship to the second access network device, andsend the second mapping relationship to the terminal via the secondaccess network device.

According to a second aspect, an embodiment of this application providesa data transmission method, including: receiving, by a first accessnetwork device, forwarded data packets from a second access networkdevice; and mapping, by the first access network device to a first DRB,at least one data packet in the forwarded data packets that includes aflow identifier, where the first DRB is corresponding to a DRB of thesecond access network device.

The first DRB and the DRB of the second access network device each meeta first mapping relationship, and the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device.

The forwarded data packets include at least one type of the followingdata packets: a PDCP layer data packet that is of the second accessnetwork device, to which a sequence number is allocated, and for whichno reception acknowledgement is obtained from a terminal; a PDCP layerdata packet that is of the second access network device and to which nosequence number is allocated; and an SDAP layer data packet of thesecond access network device.

In a possible implementation, the method further includes: mapping, bythe first access network device to the first DRB, a data packet in theforwarded data packets that includes no flow identifier.

In a possible implementation, the method further includes: mapping, bythe first access network device to a second DRB based on a secondmapping relationship, at least one data packet in the forwarded datapackets that includes a flow identifier and that is other than the datapacket mapped to the first DRB, where the second mapping relationship isa mapping relationship between a flow and a DRB in the first accessnetwork device.

In a possible implementation, the first access network device sends theforwarded data packets to the terminal on the first DRB or on the firstDRB and the second DRB.

Optionally, in the second aspect, different types of tunnels may beestablished between the first access network device and the secondaccess network device to transmit the forwarded data packets. For anexample of a specific tunnel type, refer to the related descriptions inthe first aspect. Details are not described again. Various tunnelestablishment manners are applicable to a plurality of forwarded datapacket transmission scenarios. A forwarded data packet that includes aflow identifier, a forwarded data packet that includes no flowidentifier, a forwarded data packet to which a sequence number isallocated, and a forwarded data packet to which no sequence number isallocated all can be transmitted via suitable tunnels. Therefore, apacket loss or repeated packet transmission in a data forwarding processis avoided, and network performance is improved.

In a possible implementation, the method further includes: sending, bythe first access network device to the terminal on the first DRB, a PDCPlayer data packet that is in the forwarded data packets and to which asequence number is allocated; and sending, by the first access networkdevice to the terminal on the second DRB, a PDCP layer data packet thatis in the forwarded data packets and to which no sequence number isallocated.

In a possible implementation, after sending of the forwarded datapackets mapped to the first DRB is completed, the method furtherincludes: releasing, by the first access network device, the first DRB.

In a possible implementation, the first access network device may sendthe second mapping relationship to the second access network device, andsend the second mapping relationship to the terminal via the secondaccess network device.

According to the data transmission method provided in this embodiment ofthis application, the first access network device sends the forwardeddata packets to the terminal on the first DRB corresponding to thesecond access network device. Further, to obtain better networkperformance, the first access network device establishes the second DRB.The second DRB is used to transmit the data packet in the forwarded datapackets that has a flow identifier and that is other than the datapacket mapped to the first DRB, and the second DRB meets the mappingrelationship that is between a flow and a DRB and that is configured bythe first access network device. Therefore, the forwarded data packetscan be transmitted in various flexible transmission manners on differentDRBs, and a data packet transmission manner can be selected based on anactual network status, to avoid the following problem: In a handover ordual connectivity or another scenario, a data packet is lost or isrepeatedly transmitted because each base station independentlyconfigures a mapping relationship between a flow and a DRB. Thisimproves terminal service continuity and improves communication quality.

According to a third aspect, an embodiment of this application providesa data transmission method, including: generating, by a first accessnetwork device, a forwarded data packet that includes a flow identifier;and sending, by the first access network device to a second accessnetwork device, the forwarded data packet that includes a flowidentifier.

In a possible implementation, the method further includes: sending, bythe first access network device, a first mapping relationship to thesecond access network device, where the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device.

In a possible implementation, the generating, by a first access networkdevice, a forwarded data packet that includes a flow identifierincludes:

obtaining, by the first access network device, a cache location of theforwarded data packet, where the cache location is corresponding to theflow identifier of the forwarded data packet; obtaining, by the firstaccess network device, the flow identifier of the forwarded data packetbased on the cache location; and adding, by the first access networkdevice, the flow identifier to a header of the forwarded data packet.

In a possible implementation, the method further includes: sending, bythe first access network device to the second access network device, aforwarded data packet that includes no flow identifier.

In a possible implementation, the forwarded data packets include atleast one type of the following data packets: a PDCP layer data packetthat is of the first access network device, to which a sequence numberis allocated, and for which no reception acknowledgement is obtainedfrom a terminal; a PDCP layer data packet that is of the first accessnetwork device and to which no sequence number is allocated; and an SDAPlayer data packet of the first access network device.

In a possible implementation, the forwarded data packet includes anout-of-order data packet received by the first access network devicefrom the terminal.

According to a fourth aspect, an embodiment of this application providesa data transmission method, including: receiving, by a first accessnetwork device, a forwarded data packet from a second access networkdevice, where the forwarded data packet includes a flow identifier, andthe forwarded data packet includes an out-of-order PDCP layer datapacket received by the second access network device from a terminal.

In a possible implementation, the receiving, by a first access networkdevice, a forwarded data packet from a second access network deviceincludes: receiving, by the first access network device, the forwardeddata packet from the second access network device via a tunnelestablished based on a DRB.

In a possible implementation, the method further includes: receiving, bythe first access network device, uplink data packets from the terminal,where the uplink data packets include at least one type of the followingdata packets: a PDCP layer data packet that is unsuccessfully sent bythe terminal to the second access network device and to which a sequencenumber is allocated: a PDCP layer data packet that is of the terminaland to which no sequence number is allocated: and an SDAP layer datapacket of the terminal.

In a possible implementation, the receiving, by the first access networkdevice, uplink data packets from the terminal includes: receiving, bythe first access network device, the uplink data packets on the firstDRB: or receiving, by the first access network device, the uplink datapackets on the second DRB.

In a possible implementation, the receiving, by the first access networkdevice, uplink data packets from the terminal includes: receiving, bythe first access network device on the first DRB, a PDCP layer datapacket that is of the terminal and to which a sequence number isallocated in the uplink data packets: and receiving, by the first accessnetwork device on the second DRB, the PDCP layer data packet that is ofthe terminal and to which no sequence number is allocated in the uplinkdata packets and/or the Service Data Adaptation Protocol SDAP layer datapacket of the terminal in the uplink data packets.

In a possible implementation, the receiving, by the first access networkdevice, uplink data packets from the terminal includes: receiving, bythe first access network device, a PDCP layer data packet of theterminal in the uplink data packets on the first DRB: and receiving, bythe first access network device, the Service Data Adaptation ProtocolSDAP layer data packet of the terminal in the uplink data packets on thesecond DRB.

According to the data transmission method provided in this embodiment ofthis application, in an uplink direction, the first access networkdevice receives, from the second access network device, the forwardeddata packet that includes a flow identifier. The forwarded data packetincludes the out-of-order data packet received by the second accessnetwork device from the terminal. After sending of the forwarded datapacket is completed, the terminal may send the uplink data packets to anetwork side in various flexible transmission manners on different DRBs,and a data packet transmission manner can be selected based on an actualnetwork status, to avoid the following problem: In a handover or dualconnectivity or another scenario, a data packet is lost or is repeatedlytransmitted because each base station independently configures a mappingrelationship between a flow and a DRB. This improves terminal servicecontinuity and improves communication quality.

According to a fifth aspect, an embodiment of this application providesa data transmission method, including: sending, by a terminal, uplinkdata packets to a first access network device, where the uplink datapackets include flow identifiers; and/or receiving, by the terminal,downlink data packets from the access network device, where at least oneof the downlink data packets includes a flow identifier, and thedownlink data packets include a forwarded data packet sent by a secondaccess network device to the first access network device.

In a possible implementation, the sending, by a terminal, uplink datapackets to an access network device includes: sending, by the terminalto the first access network device on a first DRB, a PDCP layer datapacket that is in the uplink data packets and to which a sequence numberis allocated: and sending, by the terminal to the first access networkdevice on a second DRB, a PDCP layer data packet to which no sequencenumber is allocated in the uplink data packets and/or an SDAP layer datapacket in the uplink data packets, where the first DRB meets a mappingrelationship between a flow and a DRB in the second access networkdevice, and the second DRB meets a mapping relationship between a flowand a DRB in the first access network device.

In a possible implementation, the sending, by a terminal, uplink datapackets to an access network device includes: sending, by the terminal,a PDCP layer data packet in the uplink data packets to the accessnetwork device on a first DRB; and sending, by the terminal, an SDAPlayer data packet in the uplink data packets to the access networkdevice on a second DRB.

In a possible implementation, the receiving, by the terminal, downlinkdata packets from the access network device includes: receiving, by theterminal from the first access network device on the first DRB, a datapacket in the downlink data packets that includes no flow identifier;and receiving, by the terminal from the first access network device onthe second DRB, a data packet in the downlink data packets that includesa flow identifier.

In a possible implementation, the receiving, by the terminal, downlinkdata packets from the access network device includes: receiving, by theterminal from the first access network device on the first DRB, a PDCPlayer data packet that is in the downlink data packets and to which asequence number is allocated; and receiving, from the first accessnetwork device on the second DRB, a PDCP layer data packet that includesa flow identifier and to which no sequence number is allocated in thedownlink data packets and/or an SDAP layer data packet in the downlinkdata packets.

In any one implementation of the fifth aspect, the first DRB meets themapping relationship between a flow and a DRB in the second accessnetwork device, and the second DRB meets the mapping relationshipbetween a flow and a DRB in the first access network device.

According to a sixth aspect, an embodiment of this application providesan access network device. The access network device has a function ofimplementing the behavior of the first access network device in any oneof the foregoing data transmission methods, or has a function ofimplementing the behavior of the second access network device in any oneof the foregoing data transmission methods. The function may beimplemented by hardware, or may be implemented by hardware by executingcorresponding software. The hardware or the software includes one ormore units or means corresponding to the function.

In a possible implementation of the sixth aspect, a structure of theaccess network device includes a processor and a transceiver. Theprocessor is configured to support the access network device inperforming the corresponding function in the foregoing data transmissionmethods. The transceiver is configured to: support communication betweenthe access network device and a terminal, and send information orinstructions in the foregoing data transmission methods to the terminal.The access network device may further include a memory. The memory isconfigured to be coupled to the processor, and the memory storesnecessary program instructions and necessary data of the access networkdevice. The access network device may further include a communicationsinterface. The communications interface is configured to communicatewith another network device.

In a possible implementation, the access network device is a basestation.

According to a seventh aspect, an embodiment of this applicationprovides a terminal. The terminal has a function of implementing thebehavior of the terminal in any one of the foregoing data transmissionmethods. The function may be implemented by hardware, or may beimplemented by hardware by executing corresponding software. Thehardware or the software includes one or more units or meanscorresponding to the function.

In a possible implementation of the seventh aspect, a structure of theterminal includes a processor and a transceiver. The processor isconfigured to support the terminal in performing the correspondingfunction in the foregoing data transmission methods. The transceiver isconfigured to: support communication between an access network deviceand the terminal, and send information or instructions in the foregoingdata transmission methods to the access network device. The terminal mayfurther include a memory. The memory is configured to be coupled to theprocessor, and the memory stores necessary program instructions andnecessary data of the terminal.

According to an eighth aspect, an embodiment of the present inventionprovides a communications system, including the access network deviceand the terminal in the foregoing aspects.

According to a ninth aspect, an embodiment of this application providesa computer readable storage medium. The computer readable storage mediumstores instructions. When the computer readable storage medium is run ona computer, the computer performs the data transmission method in anyone of the foregoing aspects.

According to a tenth aspect, an embodiment of this application providesa computer program product that includes instructions. When the computerprogram product is run on a computer, the computer performs the datatransmission method in any one of the foregoing aspects.

According to the technical solutions provided in the embodiments of thisapplication, various flexible forwarded data packet transmission mannersare used to avoid the following problem: In a handover or dualconnectivity or another scenario, a data packet is lost or is repeatedlytransmitted because each base station independently configures a mappingrelationship between a flow and a DRB. This improves terminal servicecontinuity and improves communication quality.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of thisapplication more clearly, the following briefly describes theaccompanying drawings required for describing the embodiments.Apparently, the accompanying drawings in the following descriptions showmerely some embodiments of this application, and a person of ordinaryskill in the art may still derive other drawings from these accompanyingdrawings without creative efforts.

FIG. 1 is a schematic diagram of a communications system according to anembodiment of this application;

FIG. 2 is a schematic flowchart of a data transmission method accordingto an embodiment of this application;

FIG. 3 is a schematic flowchart of a data transmission method accordingto an embodiment of this application:

FIG. 4 is a schematic flowchart of a data transmission method accordingto an embodiment of this application;

FIG. 5 is a schematic flowchart of a data transmission method accordingto an embodiment of this application;

FIG. 6 is a schematic signaling flowchart of a data transmission methodaccording to an embodiment of this application;

FIG. 7 is a schematic structural diagram of an access network device 700according to an embodiment of this application:

FIG. 8 is a schematic structural diagram of an access network device 800according to an embodiment of this application:

FIG. 9 is a schematic structural diagram of an access network device 900according to an embodiment of this application;

FIG. 10 is a schematic structural diagram of an access network device100) according to an embodiment of this application:

FIG. 11 is a schematic structural diagram of a terminal 1100 accordingto an embodiment of this application;

FIG. 12 is a schematic structural diagram of an access network device1200 according to an embodiment of this application;

FIG. 13 is a schematic structural diagram of an access network device130 according to an embodiment of this application;

FIG. 14 is a schematic structural diagram of a terminal 1400 accordingto an embodiment of this application; and

FIG. 15 is a schematic diagram of a communications system 1500 accordingto an embodiment of this application.

DESCRIPTION OF EMBODIMENTS

Technologies described in embodiments of this application may be used ina 5G (fifth generation) communications system, or anothernext-generation communications system, for example, a new radio accessnetwork (NR).

The access network device described in the embodiments of thisapplication includes a base station device in the NR, for example, agNB, a transmission point (TRP), or a base station device including acentral unit (CU) and a distributed unit (DU). The CU may also bereferred to as a control unit. When a base station device, namely, anevolved NodeB (eNB), in a Long Term Evolution (LTE) system can beconnected to a 5G core network (5G CN), the LTE eNB may also be referredto as an eLTE eNB. Specifically, the eLTE eNB is an LTE base stationdevice evolved from the LTE eNB, and can be directly connected to the 5GCN. The eLTE eNB is also a base station device in the NR. Alternatively,the access network device may be an access point (AP), or anothernetwork device capable of communicating with a terminal and a corenetwork. A type of the access network device is not specially limited inthe embodiments of this application.

The 5G CN described in the embodiments of this application may also bereferred to as a new core network, a 5G new core network, a nextgeneration core (NGC) network, or the like. The 5G CN is disposedindependently of an existing core network such as an evolved packet core(EPC) network.

The terminal in the embodiments of this application may include ahandheld device, an in-vehicle device, a wearable device, or a computingdevice that has a wireless communication function, another processingdevice connected to a wireless modem, or user equipment (UE), a mobilestation (MS), a terminal device, or the like that is in various forms.

In the embodiments of this application, a unidirectional communicationslink from an access network to the terminal is defined as a downlink,data transmitted on the downlink is downlink data, and a transmissiondirection of the downlink data is referred to as a downlink direction:and a unidirectional communications link from the terminal to the accessnetwork is an uplink, data transmitted on the uplink is uplink data, anda transmission direction of the uplink data is referred to as an uplinkdirection.

A source access network device described in the embodiments of thisapplication is an access network device that the terminal currentlyaccesses or camps on, and the terminal is to be handed over from theaccess network device to another access network device. Correspondingly,a target access network device described in the embodiments of thisapplication is an access network device to which the terminal is to behanded over.

It should be understood that the term “and/or” in this specificationdescribes only an association relationship between associated objectsand indicates that three relationships may exist. For example, A and/orB may indicate the following three cases: Only A exists, both A and Bexist, and only B exists. In addition, the character “/” in thisspecification indicates an “or” relationship between the associatedobjects.

In the embodiments of this application, “a plurality of” refers to “twoor more”.

Descriptions such as “first” and “second” in the embodiments of thisapplication are merely used to illustrate and distinguish betweendescribed objects, and are not indented to indicate a sequence orindicate a special limitation on a quantity of devices in theembodiments of this application. These descriptions cannot constituteany limitation on the embodiments of this application.

In the embodiments of this application, “connection” means variousconnection manners such as a direct connection or an indirectconnection, for implementing communication between devices. This is notlimited in the embodiments of this application.

In the embodiments of this application, “network” and “system” express asame concept, and a communications system is a communications network.

FIG. 1 is a schematic diagram of a communications system according to anembodiment of this application.

As shown in FIG. 1, the communications system includes a core networkdevice 110, a first access network device 120, a second access networkdevice 130, and a terminal 140. The first access network device 120 andthe second access network device 130 separately communicate with thecore network device 110 via communications interfaces. For example, thecommunications interface is an NG interface shown in FIG. 1. There maybe a communications interface between the first access network device120 and the second access network device 130, for example, an Xninterface shown in FIG. 1, configured to exchange information betweenthe devices.

It may be understood that the core network device 110 is a core networkdevice in a 5G CN, and includes one or more independently disposed orintegrated function entities. For example, the core network device 110may include a control plane (CP) network element and a user plane (UP)network element such as a user plane gateway (UPGW).

Optionally, the first access network device 120 or the second accessnetwork device 130 is a gNB or an eLTE eNB. This is not limited in thisembodiment of this application. For example, the first access networkdevice 120 is a gNB and the second access network device 130 is a gNB;or the first access network device 120 is an eLTE eNB and the secondaccess network device 130 is an eLTE eNB: or the first access networkdevice 120 is a gNB and the second access network device 130 is an eLTEeNB; or the first access network device 120 is an eLTE eNB and thesecond access network device 130 is a gNB.

The core network device 110 separately communicates with the firstaccess network device 120 and/or the second access network device 130via a protocol data unit (PDU) session. One PDU session may include aplurality of flows. Different flows may have a same QoS requirement ordifferent QoS requirements. The core network device 110 provides a QoSrequirement of a flow for the first access network device 120 and/or thesecond access network device 130, and the first access network device120 and/or the second access network device 130 complete/completesmapping from the flow to a DRB. Correspondingly, flows included in oneDRB have a same or similar QoS requirement. Specifically, the accessnetwork device may establish at least one DRB for each session of aterminal that accesses the access network device, including a defaultDRB. The DRB is established between the access network device and theterminal to transmit air-interface data.

In this embodiment of this application, a flow that has a specific QoSrequirement may be referred to as a QoS flow, and one QoS flow includesat least one data packet. Each QoS flow is corresponding to one or moreservice types. The QoS flow is briefly referred to as “flow” in thefollowing embodiments.

In a possible handover scenario, in a movement process, the terminal 140accesses an access network device through a handover, to obtain anoptimal communications service. For example, when the terminal 140 movesfrom a signal coverage area of the first access network device 120currently accessed by the terminal 140 to a signal coverage area of thesecond access network device 130, the terminal 140 may start a handoverprocedure to hand over from the first access network device 120 to thesecond access network device 130. In the handover process, the firstaccess network device 120 may send, to the second access network device130, a flow that the first access network device 120 is ready totransmit to the terminal 140. The second access network device 130 maymap the flow that the first access network device 120 is ready totransmit to the terminal 140 to a DRB that meets a QoS requirement ofthe flow, and then transmit the flow to the terminal on the DRB.

In a possible dual connectivity (DC) scenario, the terminal 140 accessesboth the first access network device 120 and the second access networkdevice 130. When the first access network device 120 determines tooffload some services to the second access network device 130, the firstaccess network device 120 may send a flow corresponding to the servicesto the second access network device 130. The second access networkdevice 130 may map the flow to a DRB that meets a QoS requirement of theflow, and then transmit the flow to the terminal on the DRB. Based ondifferent functions, access network devices connected to the terminalmay be classified into a master access network device that has a controlplane function and a user plane function that are related to theterminal, and a secondary access network device that can be configuredto perform user plane data transmission with the terminal. The masteraccess network device can control a service of the terminal to bemigrated between the master access network device and the secondaryaccess network device, and there is no need to distinguish between themaster access network device and the secondary access network devicewhen a flow corresponding to the service is forwarded between the accessnetwork devices and transmitted over an air interface. Therefore, thereis no need to limit the first access network device 120 and the secondaccess network device 130 to master access network devices or secondaryaccess network devices. It may be understood that the terminal mayalternatively access one master access network device and a plurality ofsecondary access network devices. Details are not described.

In the handover process or a dual connectivity process, the first accessnetwork device 120 sends data packets related to the terminal 140 to thesecond access network device 130, and then the second access networkdevice 130 transmits the data packets to the terminal 140. The datapackets belong to one or more flows. The data packet transmissionprocess may be referred to as data forwarding, or may be referred to asdata backward transmission or data transmission.

Specifically, a tunnel may be established between the first accessnetwork device 120 and the second access network device 130 to transmita data packet that needs data forwarding. The tunnel may be establishedbased on a DRB, or may be established based on an SDAP entity or asession. Alternatively, both types of tunnels may be established. Inother words, one tunnel is established based on a DRB, and the othertunnel is established based on an SDAP entity or a session. The tunnelestablished based on a DRB may be used to transmit a PDCP layer datapacket corresponding to the DRB. The tunnel established based on asession or an SDAP entity may be used to transmit a data packet cachedat an SDAP layer or transmit a PDCP layer data packet that carries aflow identifier. The SDAP layer is a user plane protocol layerestablished above a user plane PDCP layer in a protocol stack on anaccess network side connected to an NGC network. The SDAP layer may beused to map a flow from a non-access stratum (NAS) to a DRB of an accessstratum (AS). The SDAP entity is an instance that is established at theSDAP layer to complete a function of the SDAP layer. The SDAP entity isfurther responsible for adding a flow identifier in an air-interfaceprotocol stack. The flow identifier includes an uplink flow identifieror a downlink flow identifier, used to identify an uplink data flow or adownlink data flow. An access network device may map different flows toa same DRB or different DRBs based on a flow identifier of each flow anda QoS requirement of each DRB, in other words, establish a mappingrelationship between a flow and a DRB. For example, if a flow 1transmitted by the core network to the access network device is a flowcorresponding to a machine type communication (MTC) service, and a flow2 is a flow corresponding to a mobile broadband (MBB) service, it isconsidered that the access network device supports different servicetypes, and the flow 1 and the flow 2 may be mapped to a same DRB of theaccess network device, for example, a default DRB, or may be mapped totwo DRBs.

It may be understood that the SDAP entity or the SDAP layer may bereferred to as another name, for example, a Packet Data AssociationProtocol (PDAP) entity or layer. Any protocol layer that complies withthe foregoing definition and function descriptions of the SDAP layerfalls within the protection scope of the SDAP layer described in thisembodiment of this application.

Because each access network device independently sets a mappingrelationship between a flow and a DRB, after the foregoing data packetthat needs data forwarding is sent to the second access network device130, if the second access network device 130 still transmits the datapacket based on a mapping relationship that is between a flow and a DRBand that is configured by the first access network device 120, the datapacket may be lost or repeatedly transmitted. This affects terminalservice continuity.

Therefore, the embodiments of this application provide a datatransmission method, to resolve the following problem about how toforward data and how to transmit, over an air interface, a data packetthat needs data forwarding, on a premise that each access network deviceindependently sets a mapping relationship between a flow and a DRB,especially in a handover or dual connectivity scenario.

For ease of description, “first access network device” or “second accessnetwork device” has a same meaning in the following embodiments. Detailsare not described below again. For example, in a handover process, thefirst access network device in the embodiments of this application maybe a target access network device, and the second access network devicemay be a source access network device. In a dual connectivity scenario,the second access network device in the embodiments of this applicationmay offload some services to the first access network device, and thefirst access network device transmits the services to a terminal. Forexample, when a master cell group bearer (MCG bearer) is used, thesecond access network device is a master base station and the firstaccess network device is a secondary base station; or when a secondarycell group bearer (SCG bearer) is used, the second access network deviceis a secondary base station and the first access network device is amaster base station.

FIG. 2 is a schematic flowchart of a data transmission method accordingto an embodiment of this application.

The data transmission method provided in this application is applicableto various communication scenarios that have a process of forwardingdata between base stations, such as a handover process or a dualconnectivity process of a terminal. This is not limited in thisembodiment of this application.

The method includes the following steps.

S201. A first access network device receives forwarded data packets froma second access network device.

The forwarded data packets are data packets that need data forwardingand that are sent by the second access network device to the firstaccess network device, namely, data packets sent by the second accessnetwork device to the first access network device in a data forwardingprocess.

Specifically, in a downlink direction, the forwarded data packetsinclude at least one type of the following data packets: a PDCP layerdata packet of the second access network device, to which a sequencenumber (SN) is allocated, and for which no reception acknowledgement isobtained from a terminal: a PDCP layer data packet of the second accessnetwork device to which no sequence number is allocated: and an SDAPlayer data packet of the second access network device.

PDCP layer data packets include a PDCP PDU and a PDCP SDU. When thesecond access network device forwards the PDCP PDU to the first accessnetwork device, the second access network device may perform processingsuch as decryption or protocol header removal on the PDCP PDU, to obtaina PDCP SDU whose sequence number is retained. Therefore, all PDCP layerdata packets are forwarded between base stations in a form of PDCP SDUs,including a PDCP SDU to which a sequence number is allocated and a PDCPSDU to which no sequence number is allocated. In this embodiment of thisapplication, the sequence number of the PDCP SDU is a PDCP layersequence number that may be denoted as a PDCP SN.

S202. The first access network device maps, to a first DRB, at least onedata packet in the forwarded data packets that includes a flowidentifier, where the first DRB is corresponding to a DRB of the secondaccess network device.

The first access network device may map, to the first DRB, all datapackets including flow identifiers in the forwarded data packets or somedata packets including flow identifiers in the forwarded data packets.

The first DRB and the DRB of the second access network device each meeta first mapping relationship, and the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device. Specifically, the first DRB is established by the firstaccess network device and is used to transmit data between the firstaccess network device and the terminal. When the first access networkdevice receives information about one or more flows of the DRB of thesecond access network device, the first access network device mayestablish a reflect DRB. The reflect DRB can maintain a transmissionstatus of a specific DRB of the second access network device, tocontinue to transmit a data packet on the DRB of the second accessnetwork device. For ease of description, a DRB of the second accessnetwork device corresponding to the reflect DRB may be referred to as“third DRB”. Specifically, the reflect DRB has a same PDCP SN status andhyper frame number (HFN) status as the third DRB. The PDCP SN status andthe HFN status may indicate a sending status and a receiving status of aPDCP data packet on the DRB. The reflect DRB is the foregoing “firstDRB”.

Optionally, the first access network device receives the first mappingrelationship from the second access network device.

Optionally, the first access network device maps, to the first DRB, adata packet in the forwarded data packets that includes no flowidentifier.

After the first access network device maps some or all of the forwardeddata packets to the first DRB, the first access network device sends thecorresponding data packets to the terminal on the first DRB, forexample, sends, on the first DRB, some or all data packets includingflow identifiers in the forwarded data packets and all data packetsincluding no flow identifier in the forwarded data packets.

Optionally, in an implementation of this application, the method furtherincludes: mapping, by the first access network device, to a second DRBbased on a second mapping relationship, at least one data packet in theforwarded data packets including a flow identifier other than the datapacket mapped to the first DRB, where the second mapping relationship isa mapping relationship between a flow and a DRB in the first accessnetwork device.

The second mapping relationship may include a correspondence between aflow identifier of each flow and a DRB in the first access networkdevice. The first access network device may configure the mappingrelationship between a flow and a DRB based on a QoS requirement, andestablish the second DRB based on the mapping relationship. Such a DRBestablished by an access network device based on a mapping relationshipthat is configured by the access network device between a flow and a DRBmay also be referred to as a new DRB. The QoS requirement includes a QoSparameter. When a handover between access network devices is performedvia a direct interface, the QoS parameter may be configured by a sourceaccess network device and sent by the source access network device to atarget access network device. When a handover between access networkdevices is performed via a core network, the QoS parameter may be sentfrom a source base station to a core network device, and then sent bythe core network device to a target access network device. The corenetwork device may modify the QoS parameter.

The first access network device may separately send, on the first DRBand the second DRB, the data packets including flow identifiers in theforwarded data packets, for example, send, to the terminal on the firstDRB, some data packets that include flow identifiers, and send, to theterminal on the second DRB, the other data packets that include flowidentifiers. Specifically, the first access network device may send, tothe terminal on the first DRB, a PDCP layer data packet including a flowidentifier and to which a sequence number is allocated in the forwardeddata packets, and send, to the terminal on the second DRB, a PDCP layerdata packet including a flow identifier and to which no sequence numberis allocated in the forwarded data packets.

Specifically, an SDAP entity of the first access network device mayseparately route, to different DRBs, the data packets that include flowidentifiers.

It may be understood that the second DRB may be the same as or differentfrom the first DRB. If the first mapping relationship is the same as thesecond mapping relationship, the first DRB is the same as the secondDRB. Specifically, a DRB of the first access network device may be firstused as a reflect DRB to send a received forwarded data packet, and thenused as a new DRB to send a data packet received from a core network.The same DRB is divided into the reflect DRB and the new DRB in a timedimension. In other words, different mapping relationships between aflow and a DRB may be used for different data packets received atdifferent times. If the first mapping relationship is different from thesecond mapping relationship, the first DRB and the second DRB may be twoindependently established DRBs.

Optionally, in an implementation of this application, if a quantity ofsecond DRBs is less than a quantity of first DRBs, and the first DRB isdifferent from the second DRB, after data packet sending on the firstDRB is completed, the first access network device and the terminal mayrelease the first DRB, so that overheads of the terminal and the firstaccess network device can be reduced. Specifically, the first accessnetwork device may notify the terminal to release the first DRB. Theterminal receives, from the first access network device, a notificationmessage for instructing the terminal to release the first DRB, andreleases a configuration of the first DRB. After receiving thenotification message, the terminal can acknowledge that downlink datapacket sending on the first DRB is completed. The notification messagemay be considered as an end marker, used to indicate that downlink datapacket transmission on the first DRB ends.

Optionally, the first access network device may send the second mappingrelationship to the second access network device, and the second accessnetwork device sends the second mapping relationship to the terminal.

Optionally, in an implementation of this application, the first accessnetwork device receives the forwarded data packets via a tunnel betweenthe first access network device and the second access network device. Ina downlink direction, the tunnel between the first access network deviceand the second access network device may be established in differentmanners. A tunnel establishment manner is not specially limited in thisembodiment of this application.

For example, the tunnel is a tunnel established based on a DRB. Thetunnel established based on a DRB may be established between the thirdDRB of the second access device and the reflect DRB of the first accessdevice, or may be established between the third DRB and the new DRB ofthe first access device. One or more DRB-based tunnels may beestablished between the first access network device and the secondaccess network device. The tunnel established based on a DRB may be usedto transmit the PDCP layer data packet.

Optionally, the first access network device may map, to the first DRB, adata packet including no flow identifier that is received via the tunnelestablished based on a DRB, and map, to the second DRB, a data packetincluding a flow identifier that is received via the tunnel establishedbased on a DRB.

For another example, the tunnel is a tunnel established based on asession. The tunnel established based on a session may also be referredto as a tunnel established based on an SDAP entity, and the tunnel isestablished between the SDAP entity of the first access device and anSDAP entity of the second access network device that are correspondingto a same session.

The tunnel established based on a session may be used to transmit a datapacket carrying a flow identifier to which no sequence number isallocated in the forwarded data packets on all DRBs in the session.

Optionally, the first access network device routes, to the first DRB viathe SDAP entity, a PDCP layer data packet that is received from thetunnel established based on a session and to which a sequence number isallocated, and routes, to the second DRB, a PDCP layer data packet towhich no sequence number is allocated or the SDAP layer data packet.

For another example, the tunnel includes a tunnel established based on aDRB and a tunnel established based on a session.

Optionally, the tunnel established based on a DRB is used to transmitthe PDCP layer data packet, and the tunnel established based on asession is used to transmit the SDAP layer data packet.

Optionally, the tunnel established based on a DRB is used to transmit adata packet that is cached at a PDCP layer of the second access networkdevice, and to which a sequence number is allocated. The tunnelestablished based on a session is used to transmit the data packet inthe forwarded data packets that carries a flow identifier, including theSDAP layer data packet of the second access network device, and/or adata packet carrying a flow identifier to which no sequence number isallocated that is cached at the PDCP layer of the second access networkdevice.

Optionally, the first access network device may map, to the first DRB, adata packet received via the tunnel established based on a DRB, and map,to the second DRB, a data packet received via the tunnel establishedbased on a session.

In this embodiment of this application, the SDAP layer data packetincludes a data packet cached at the SDAP layer, and the PDCP layer datapacket includes a data packet cached at the PDCP layer.

It may be understood that the first access network device maysimultaneously establish a tunnel and a DRB. For example, in a handoverscenario, if a handover request message sent by the second accessnetwork device to the first access network device includes informationrequired for establishing a tunnel and information required forestablishing a DRB, the first access network device can perform acorresponding operation after receiving the related information.

According to the data transmission method provided in this embodiment ofthis application, the first access network device receives the forwardeddata packets from the second access network device, and the first accessnetwork device may map, to the first DRB established by the first accessnetwork device, the at least one data packet in the forwarded datapackets that includes a flow identifier. The first DRB is correspondingto the DRB of the second access network device. Further, to obtainbetter network performance, the first access network device establishesthe second DRB. The second DRB is used to transmit the data packethaving a flow identifier in the forwarded data packets other than thedata packet mapped to the first DRB, and the second DRB meets themapping relationship that is between a flow and a DRB and that isconfigured by the first access network device. Therefore, the forwardeddata packets can be transmitted in various flexible transmission mannerson different DRBs. and a data packet transmission manner can be selectedbased on an actual network status, to avoid the following problem: In ahandover or dual connectivity or another scenario, a data packet is lostor is repeatedly transmitted because each base station independentlyconfigures a mapping relationship between a flow and a DRB. Thisimproves terminal service continuity and improves communication quality.

FIG. 3 is a schematic flowchart of a data transmission method accordingto an embodiment of this application.

Similar to the data transmission method provided in the embodiment shownin FIG. 2, the data transmission method provided in this embodiment isapplicable to various scenarios that have a process of forwarding databetween base stations, such as a handover process or a dual connectivityprocess of a terminal. Details are not described.

The method includes the following steps.

S301. A first access network device receives forwarded data packets (ora forwarded data packet) from a second access network device.

For detailed descriptions of the forwarded data packets, refer to therelated content in the embodiment shown in FIG. 2. Details are notdescribed herein again.

S302. The first access network device maps, to a first DRB, a datapacket in the forwarded data packets that includes no flow identifier,where the first DRB is corresponding to a DRB of the second accessnetwork device (briefly referred to as “third DRB”).

Specifically, the first DRB and the third DRB each meet a first mappingrelationship, and the first mapping relationship is a mappingrelationship between a flow and a DRB in the second access networkdevice.

Optionally, the first access network device receives the first mappingrelationship from the second access network device.

S303. The first access network device maps, to a second DRB based on asecond mapping relationship, a data packet in the forwarded data packetsthat includes a flow identifier, where the second mapping relationshipis a mapping relationship between a flow and a DRB in the first accessnetwork device.

The first DRB is a reflect DRB, and the second DRB is a new DRB. Forrelated detailed descriptions, refer to the related content in theembodiment shown in FIG. 2. Details are not described herein again. Itmay be understood that the first DRB and the second DRB are separatelyestablished by the first access network device, and there is noexecution sequence of steps of establishing the first DRB andestablishing the second DRB by the first access network device.

For detailed descriptions of the first mapping relationship and thesecond mapping relationship, refer to the related content in theembodiment shown in FIG. 2. Details are not described herein again.

Optionally, the first access network device routes, via an SDAP entityto the second DRB, the data packet in the forwarded data packets thatincludes a flow identifier.

It may be understood that there is no execution sequence of step S302and step S303. For example, S302 may be performed before S303, or S303may be performed before S302, or the two steps S302 and S303 may besimultaneously performed. This is not specially limited in thisembodiment of this application.

Optionally, in an implementation of this application, the method furtherincludes: sending, by the first access network device to a terminal onthe first DRB, a data packet in the forwarded data packets that includesno flow identifier and to which a sequence number is allocated; andsending, by the first access network device to the terminal on thesecond DRB, a data packet in the forwarded data packets that includes aflow identifier and to which no sequence number is allocated. The datapacket may be a PDCP layer data packet, and the sequence number is aPDCP SN.

Optionally, in an implementation of this application, the method furtherincludes: sending, by the first access network device to a terminal onthe first DRB, a data packet in the forwarded data packets that includesno flow identifier; and sending, by the first access network device tothe terminal on the second DRB, a data packet in the forwarded datapackets that includes a flow identifier.

Optionally, in an implementation of this application, if a quantity ofsecond DRBs is less than a quantity of first DRBs, and the first DRB isdifferent from the second DRB, after data packet sending on the firstDRB is completed, the first access network device and the terminal mayseparately release the first DRB, so that overheads of the terminal andthe first access network device can be reduced. For related detaileddescriptions, refer to the related content in the embodiment shown inFIG. 2. Details are not described herein again.

Optionally, the first access network device may send the second mappingrelationship to the second access network device, and the second accessnetwork device sends the second mapping relationship to the terminal.

Optionally, in an implementation of this application, the first accessnetwork device receives the forwarded data packets via a tunnel betweenthe first access network device and the second access network device. Ina downlink direction, the tunnel between the first access network deviceand the second access network device may be established in differentmanners.

For example, the first access network device receives the forwarded datapackets from the second access network device via a tunnel establishedbased on a DRB and a tunnel established based on a session.

Optionally, the tunnel established based on a DRB is used to transmit adata packet that is cached at a PDCP layer of the second access networkdevice, and to which a sequence number is allocated. The tunnelestablished based on a session is used to transmit the data packet inthe forwarded data packets that carries a flow identifier, including anSDAP layer data packet of the second access network device, and/or adata packet that is cached at the PDCP layer of the second accessnetwork device, that carries a flow identifier, and to which no sequencenumber is allocated.

For another example, the first access network device receives theforwarded data packets from the second access network device via atunnel established based on a DRB.

For another example, the first access network device receives, from thesecond access network device via a tunnel established based on asession, the data packet in the forwarded data packets that includes aflow identifier.

Optionally, the first access network device routes, to the first DRB viathe SDAP entity, a PDCP layer data packet that is received from thetunnel established based on a session and to which a sequence number isallocated, and routes, to the second DRB, a PDCP layer data packet towhich no sequence number is allocated or an SDAP layer data packet.

For detailed descriptions of the foregoing various tunnels, refer to therelated content in the embodiment shown in FIG. 2. Details are notdescribed herein again.

Optionally, after sending of the forwarded data packet mapped to thefirst DRB is completed, the first access network device may release thefirst DRB, to save a resource. For detailed descriptions of releasingthe first DRB, refer to the related content in the embodiment shown inFIG. 2. Details are not described herein again.

According to the data transmission method provided in this embodiment ofthis application, on a premise that each access network device canindependently set a mapping relationship between a flow and a DRB, thedata packet that has a flow identifier is mapped to the second DRB fortransmission, and the data packet that has no flow identifier is mappedto the first DRB for transmission. Therefore, a data packet transmissionmanner can be selected based on an actual network status, to avoid thefollowing problem: In a handover or dual connectivity or anotherscenario, a data packet is lost or is repeatedly transmitted becauseeach access network device independently configures a mappingrelationship between a flow and a DRB. This improves terminal servicecontinuity and improves communication quality.

FIG. 4 is a schematic flowchart of a data transmission method accordingto an embodiment of this application.

The data transmission method provided in this embodiment is applicableto various scenarios that have a process of forwarding data between basestations, such as a handover process or a dual connectivity process of aterminal. Details are not described.

The method includes the following steps.

S401. A first access network device receives a forwarded data packet (orforwarded data packets) from a second access network device, where theforwarded data packet includes a flow identifier, and the forwarded datapacket includes an out-of-order data packet received by the secondaccess network device from a terminal.

Specifically, in an uplink direction, the forwarded data packet is anout-of-order PDCP layer data packet received by the second accessnetwork device from the terminal. For example, if a sequence number of alast PDCP SDU sequentially received by the second access network deviceis an SN, in other words, PDCP SDUs whose sequence numbers ( . . . ,SN−1, SN) are less than the SN have been sequentially received, anout-of-order PDCP SDU that is received by the second access networkdevice and whose sequence number is greater than the SN is a data packetthat needs data forwarding. For example, data forwarding is required forPDCP SDUs whose sequence numbers are SN+3, SN+4, and SN+6 and that arereceived by the second access network device after receiving the PDCPSDU with the sequence number being the SN.

S402. The first access network device sends the received forwarded datapacket (or forwarded data packets) to a core network device.

Optionally, the first access network device receives the forwarded datapacket via a tunnel between the first access network device and thesecond access network device. The tunnel may be a tunnel that isestablished between the first access network device and the secondaccess network device based on a DRB. For the tunnel, refer to therelated descriptions of the tunnel established based on a DRB in anotherembodiment of this application. Details are not described again.

Optionally, in an implementation of this application, the method furtherincludes: receiving, by the first access network device, uplink datapackets from the terminal, where the uplink data packets include atleast one type of the following data packets: a PDCP layer data packetthat is unsuccessfully sent by the terminal to the second access networkdevice and to which a sequence number is allocated; a PDCP layer datapacket of the terminal, to which no sequence number is allocated; and anSDAP layer data packet of the terminal.

Optionally, after the terminal completes uplink data packet sending on afirst DRB, the terminal may request the first access network device torelease the first DRB. The first access network device may consider thata request sent by the terminal to release the first DRB is an endmarker, and the request is used to indicate that uplink data packettransmission on the first DRB ends.

After accessing the first access network device, the terminal may sendthe uplink data packets to the first access network device in differentsending manners.

Optionally, the first access network device receives the uplink datapackets on the first DRB.

Optionally, the first access network device receives the uplink datapackets on a second DRB.

Optionally, the first access network device receives, on the first DRB,a PDCP layer data packet of the terminal, to which a sequence number isallocated in the uplink data packets; and the first access networkdevice receives, on a second DRB, the PDCP layer data packet of theterminal, to which no sequence number is allocated in the uplink datapackets and/or the SDAP layer data packet of the terminal in the uplinkdata packets.

Optionally, the first access network device receives a PDCP layer datapacket of the terminal in the uplink data packets on the first DRB; andthe first access network device receives the SDAP layer data packet ofthe terminal in the uplink data packets on a second DRB.

The first DRB is a reflect DRB, and the second DRB is a new DRB. Forrelated detailed descriptions, refer to the related content in anotherembodiment of this application. Details are not described herein again.

According to the data transmission method provided in this embodiment ofthis application, in the uplink direction, the first access networkdevice receives, from the second access network device, the forwardeddata packet that includes a flow identifier. The forwarded data packetincludes the out-of-order data packet received by the second accessnetwork device from the terminal. After sending of the forwarded datapacket is completed, the terminal may send the uplink data packets to anetwork side in various flexible transmission manners on different DRBs,and a data packet transmission manner can be selected based on an actualnetwork status, to avoid the following problem: In a handover or dualconnectivity or another scenario, a data packet is lost or is repeatedlytransmitted because each base station independently configures a mappingrelationship between a flow and a DRB. This improves terminal servicecontinuity and improves communication quality.

FIG. 5 is a schematic flowchart of a data transmission method accordingto an embodiment of this application.

Similar to the data transmission method provided in the embodiment shownin FIG. 2, the data transmission method provided in this embodiment isapplicable to various scenarios that have a process of forwarding databetween base stations, such as a handover process or a dual connectivityprocess of a terminal. Details are not described.

The method includes the following steps.

S501. A second access network device generates a data packet forforwarding that includes a flow identifier (or data packets forforwarding that include flow identifiers).

S502. The second access network device sends, to a first access networkdevice, the data packet for forwarding that includes a flow identifier(or data packets for forwarding that include flow identifiers).

Optionally, in a downlink direction, the data packet for forwardingincludes at least one type of the following data packets: a PDCP layerdata packet of the second access network device, to which a sequencenumber is allocated, and for which no reception acknowledgement isobtained from a terminal: a PDCP layer data packet of the second accessnetwork device to which no sequence number is allocated; and a ServiceData Adaptation Protocol SDAP layer data packet of the second accessnetwork device.

Optionally, in the downlink direction, the second access network devicefurther sends, to the first access network device, a data packet forforwarding that includes no flow identifier (or data packets forforwarding include no flow identifier).

Optionally, in the downlink direction, the second access network devicemay send the data packets for forwarding to the first access networkdevice via different types of tunnels. For detailed descriptions ofvarious tunnels and transmission of the data packets for forwarding inthe various tunnels, refer to the related content in another embodimentof this application. Details are not described herein again.

It may be understood that in the downlink direction, the first accessnetwork device may send, to the terminal, downlink data packets thatinclude the forwarded data packet. Specifically, after the second accessnetwork device sends the data packet for forwarding to the first accessnetwork device, the first access network device may map the forwardeddata packet to a corresponding DRB based on a factor such as content(for example, whether the forwarded data packet has a flow identifier,or whether a sequence number is allocated to the forwarded data packet)of the forwarded data packet or a type of a tunnel that carries theforwarded data packet, to transmit the forwarded data packet. Fordetailed descriptions of mapping the forwarded data packet to thecorresponding DRB by the first access network device, refer to therelated content in another embodiment of this application. Details arenot described herein again. Then, the first access network device sends,to the terminal, another downlink data packet obtained from a corenetwork.

Optionally, in an implementation of this application, in the downlinkdirection, a PDCP entity of the second access network device obtains theflow identifier, so that the second access network device may add theflow identifier to the data packet for forwarding, and generate the datapacket for forwarding that includes the flow identifier, and then thefirst access network device parses the forwarded data packet to obtainthe flow identifier.

Optionally, the second access network device obtains the flow identifierof the data packet based on a correspondence between a service accesspoint (SAP) and the flow identifier. Specifically, the second accessnetwork device establishes one or more SAPs between an SDAP entity and aPDCP entity, and the PDCP entity separately caches a PDCP PDU or a PDCPSDU based on an SAP and a flow identifier. Each SAP is corresponding toone flow. When forwarding data, the second access network device maysend the PDCP PDU or the PDCP SDU to the SDAP entity based on the SAP,and then the SDAP entity may obtain the flow identifier of the PDCPlayer data packet based on a correspondence between the SAP and a flow.Alternatively, the PDCP entity may obtain the flow identifier based onSAP information. The forwarded data packet includes a PDCP PDU and aPDCP SDU, and a PDCP SDU that carries a sequence number is generatedafter processing such as protocol header removal or decryption isperformed on the PDCP PDU.

Optionally, the second access network device obtains the flow identifierof the data packet based on a cache location of the data packet.Specifically, an SDAP entity of the second access network device sendsthe flow identifier of the data packet to a PDCP entity, and the PDCPentity establishes a correspondence between the flow identifier and thecache location of the data packet. For example, the PDCP entity maycache the received PDCP SDU based on the flow identifier. Then, thesecond access network device obtains the cache location of the forwardeddata packet. The cache location of the forwarded data packet iscorresponding to the flow identifier of the forwarded data packet. Thesecond access network device obtains the flow identifier of the datapacket based on the cache location of the data packet. Optionally, anSDAP layer of the second access network device further includesindication information, and the indication information is used toindicate whether a PDCP layer adds a flow identifier to a PDCP PDU andsends, over an air interface, the PDCP PDU that includes the flowidentifier. For example, if the indication information indicates that noflow identifier is added, the PDCP PDU is generated by removing data ofthe flow identifier from an SDAP header. Because the PDCP PDU includesno flow identifier, data overheads are reduced.

Optionally, in a handover preparation process, an SDAP entity of thesecond access network device may add a flow identifier to a receiveddata packet, and indicate that the flow identifier is used only for ahandover process. Therefore, the first access network device may restorethe flow identifier based on the received forwarded data packet.

Optionally, in a handover preparation process, an SDAP entity of thesecond access network device may add a flow identifier to a receiveddata packet. For example, when the second access network device freezesa transmission status, the SDAP entity starts to add flow identifiers toall data packets to be sent to a PDCP layer, and the transmission statusfreezing means that the second access network device no longer sendsdata to the terminal. Therefore, the first access network device mayrestore the flow identifier based on the received forwarded data packet.

Optionally, the second access network device determines the flowidentifier based on a sequence number included in the forwarded datapacket. Specifically, an SDAP entity of the second access network devicecaches the data packet, configures an SDAP sequence number for thecached data packet, and sends, to a PDCP entity, the data packet forwhich the SDAP sequence number is configured. If the PDCP entitysuccessfully sends, to the first access network device, a data packetthat includes a PDCP SN, the PDCP entity sends an indication to an SDAPlayer. After receiving the indication, the SDAP layer deletes a datapacket corresponding to the data packet that includes the PDCP SN. Adata packet cached at the SDAP layer has a flow identifier, a datapacket sent by the SDAP entity to the PDCP entity includes an SDAPsequence number, and the SDAP sequence number is corresponding to theflow identifier. Therefore, the SDAP entity of the second access networkdevice obtains the flow identifier of the data packet through matchingbased on the SDAP sequence number of the data packet received from thePDCP layer.

Optionally, in an uplink direction, the forwarded data packet includesan out-of-order data packet received by the second access network devicefrom the terminal. All forwarded data packets in the uplink directioninclude flow identifiers. The second access network device may send theforwarded data packet in the uplink direction to the first accessnetwork device via a tunnel established based on a DRB. Details are notdescribed.

It may be understood that in the uplink direction, after the secondaccess network device sends the forwarded data packet to the firstaccess network device, the first access network device may send theforwarded data packet to the core network. After accessing the firstaccess network device, the terminal may send uplink data packets to thefirst access network device on different DRBs. For detailed descriptionsof sending, by the terminal, the uplink data packets to the first accessnetwork device, refer to the related content in another embodiment ofthis application. Details are not described herein again.

FIG. 6 is a schematic signaling flowchart of a data transmission methodaccording to an embodiment of this application.

In the embodiment shown in FIG. 6, an example in which a terminal ishanded over from a source base station to a target base station is usedto describe the data transmission method provided in this embodiment ofthis application. It may be understood that the embodiment shown in FIG.6 is a further explanation and description of the embodiments shown inFIG. 2 to FIG. 5. The source base station is an example of the secondaccess network device in the embodiments shown in FIG. 2 to FIG. 5. Thetarget base station is an example of the first access network device inthe embodiments shown in FIG. 2 to FIG. 5. Mutual reference may be madebetween the embodiments provided in this application.

The method includes the following steps.

S601. A terminal sends a measurement report to a source base station.

S602. The source base station determines, based on the receivedmeasurement report, to trigger a handover procedure.

S603. The source base station sends a first mapping relationship to atarget base station.

The first mapping relationship is a mapping relationship that is betweena flow and a DRB and that is configured by the source base station. Fordetailed descriptions of the first mapping relationship, refer to therelated content of another embodiment of this application. Details arenot described herein again.

Optionally, the first mapping relationship is included in signaling or amessage that is sent by the source base station to the target basestation in a handover process, for example, a handover request message.This is not limited in this embodiment of this application.

Optionally, after the target base station receives the handover requestmessage sent by the source base station, the target base station mayestablish a DRB between the target base station and the terminal,including a reflect DRB (namely, the first DRB in the embodiment shownin FIG. 2 or FIG. 3) for maintaining a transmission status of a DRB ofthe source base station. In addition to the reflect DRB, the target basestation may further establish a new bearer (namely, the second DRB inthe embodiment shown in FIG. 2 or FIG. 3) based on a mappingrelationship that is between a flow and a DRB and that is configured bythe target base station. For detailed descriptions of the reflect DRBand the new DRB, refer to the related content in another embodiment ofthis application. Details are not described herein again.

S604. The target base station sends a second mapping relationship to thesource base station.

The second mapping relationship may be included in signaling or amessage that is sent by the target base station to the source basestation in the handover process, for example, a handover requestresponse message. This is not limited in this embodiment of thisapplication.

The second mapping relationship is a mapping relationship that isbetween a flow and a DRB and that is configured by the target basestation. For detailed descriptions of the second mapping relationship,refer to the related content of another embodiment of this application.Details are not described herein.

S605. The source base station sends a handover command to the terminal.

The handover command may include the second mapping relationship, sothat the terminal may obtain the second mapping relationship.

The second mapping relationship may be the same as or different from thefirst mapping relationship. Specifically, because each access networkdevice independently configures a mapping relationship between a DRB anda flow based on a QoS requirement of the flow, the mapping relationshipthat is between a DRB and a flow and that is configured by the targetbase station may be the same as or different from the mappingrelationship that is between a DRB and a flow and that is configured bythe source base station.

Optionally, the response message and the handover command furtherinclude a third mapping relationship, and the third mapping relationshipis a correspondence between the reflect DRB configured by the targetbase station and the DRB configured by the target base station.

S606. The source base station sends an air-interface transmission statusof a PDCP layer to the target base station.

The air-interface transmission status of the PDCP layer means a sendingstatus and a receiving status of a PDCP layer data packet on the DRB ofthe source base station. A transmission status of an uplink data packetincludes a sequence number of a first lost PDCP SDU, and a receivingstatus of a PDCP SDU between the first lost PDCP SDU and a last receivedPDCP SDU. The receiving status specifically means whether a data packetis received. A transmission status of a downlink data packet includes asequence number of a next PDCP SDU to which the target base stationneeds to allocate a sequence number, and the sequence number includes aPDCP SN and an HFN.

S607. The source base station sends data packets for forwarding (or adata packet for forwarding) to the target base station.

The data packets for forwarding include a data packet for forwarding inan uplink direction, and/or a data packet for forwarding in a downlinkdirection. For detailed descriptions of the forwarded data packets,refer to the related content in the embodiments shown in FIG. 2 to FIG.4. Details are not described herein again.

Specifically, the source base station may send the data packets forforwarding to the target base station via a tunnel between the sourcebase station and the target base station. For various tunnels andspecific tunnel establishment manners, refer to the related content inthe embodiments shown in FIG. 2 to FIG. 5. Details are not describedherein again.

In this embodiment, a PDCP entity of the source base station obtains aflow identifier of the data packet, so that the source base station mayadd the flow identifier to the data packet for forwarding, and then thetarget base station may map the forwarded data packet to a correspondingDRB based on the flow identifier. For detailed descriptions ofgenerating, by the source base station, the data packet that include theflow identifier, refer to the related content in the embodiment shown inFIG. 5. Details are not described herein again.

S608. The terminal accesses the target base station.

S609. The target base station performs path switching.

Optionally, the method further includes: notifying, by the target basestation, a control plane management network element of a core network tonotify a user plane network element to send a subsequent data packetrelated to the terminal to the target base station.

Optionally, in an implementation of this application, after the terminalis handed over to the target base station, the method further includes:S610. The terminal performs data communication with the target basestation.

The data communication includes uplink data transmission and/or downlinkdata transmission.

Specifically, in an uplink data transmission process or a downlink datatransmission process, forwarded data packets received via differenttypes of tunnels are transmitted in different manners. This is notspecially limited in this embodiment of this application. In addition,in the downlink data transmission process, the target base station maysend, to the terminal, the forwarded data packet in the downlinkdirection and another downlink data packet received from the corenetwork. In the uplink data transmission process, the target basestation may send, to the core network, the forwarded data packet in theuplink direction and another uplink data packet received from theterminal. Transmission of the forwarded data packet takes precedenceover that of the another uplink data packet received from the terminalor the another downlink data packet received from the core network.Optionally, the uplink data packet and the downlink data packet may becarried on a same DRB. In other words, a bidirectional DRB provides anuplink service and a downlink service. Alternatively, the uplink datapacket and the downlink data packet may be carried on different DRBs.

In the downlink direction, the target base station may send, to theterminal on the first DRB or the second DRB, downlink data packets thatinclude the forwarded data packets.

For example, when the data packets for forwarding are sent to the targetbase station via one DRB-based tunnel and one tunnel established basedon a session, a specific manner in which the target base station sendsthe data packets to the terminal may include the following:

If the second mapping relationship is the same as the first mappingrelationship, the target base station may send the forwarded datapackets to the terminal on the first DRB, and then, after completingsending of the forwarded data packets, send, still on the first DRB,another downlink data packet received from the core network via an SDAPlayer.

If the second mapping relationship is different from the first mappingrelationship, the target base station may send, to the terminal on thefirst DRB, a data packet received by the target base station via thetunnel established based on a DRB; and send, to the terminal on thesecond DRB, a data packet received by the target base station via thetunnel established based on a session.

Optionally, the target base station sends, to the terminal on the secondDRB, a data packet in the forwarded data packets that includes a flowidentifier, and the target base station sends, to the terminal on thefirst DRB, a data packet in the forwarded data packets that includes noflow identifier.

Optionally, the target base station may send, to the terminal on thefirst DRB, a PDCP SDU in the forwarded data packets, to which a sequencenumber is allocated, and the target base station sends, to the terminalon the second DRB, a PDCP SDU that includes a flow identifier and towhich no sequence number is allocated. The target base station mayfurther send an SDAP layer data packet of the source base station to theterminal on the second DRB. It may be understood that the first DRB maybe used to transmit a data packet that has a flow identifier or a datapacket that has no flow identifier. Therefore, any PDCP SDU that has asequence number can be transmitted on the first DRB regardless ofwhether the PDCP SDU includes a flow identifier.

Optionally, the source base station may allocate a sequence number to aPDCP SDU that is cached at the PDCP layer and to which no sequencenumber is allocated, and send, to the target base station as some of thedata packets for forwarding via the tunnel established based on a DRB,the PDCP SDU to which the sequence number is allocated. Then, in asubsequent communication process, the target base station may directlytransmit, on the first DRB, the PDCP SDU to which the sequence number isallocated, without allocating a sequence number to the PDCP SDU. Thissimplifies a procedure and improves transmission efficiency.

For another example, when the data packets for forwarding are sent tothe target base station via a tunnel established based on a session, thetarget base station may route, to the first DRB via an SDAP entity, aPDCP layer data packet in the data packets to which a sequence number isallocated, and send, to the terminal on the first DRB, the PDCP layerdata packet to which the sequence number is allocated. The target basestation routes, to the second DRB via the SDAP entity, remaining datapackets such as a data packet cached at an SDAP layer and/or a PDCPlayer data packet to which no sequence number is allocated, and sendsthe remaining data packets to the terminal on the second DRB.Optionally, the SDAP entity may determine a plurality of second DRBsbased on the mapping relationship between a flow and a DRB in the targetbase station, and send the remaining data packets on the plurality ofsecond DRBs, to transmit the remaining data packets to the terminal. ThePDCP layer data packet in the forwarded data packets to which thesequence number is allocated may carry an identifier of a third DRB, andthe target base station may forward, to the corresponding first DRBbased on a mapping relationship between the third DRB and the first DRB,the received data packet to which the sequence number is allocated.

For another example, when the data packets for forwarding are sent tothe target base station via one or more tunnels established based on aDRB, a specific manner in which the target base station sends the datapackets to the terminal includes the following:

If the first mapping relationship is the same as the second mappingrelationship, the target base station sends, to the terminal on thefirst DRB, the data packets received by the target base station via thetunnel established based on a DRB, and then, after completing sending ofthe forwarded data packets, sends, by still using the first DRB, anotherdata packet received from the core network via an SDAP layer.

Optionally, the source base station may allocate a sequence number to aPDCP SDU that is cached at the PDCP layer and to which no sequencenumber is allocated, and send, to the target base station as some of thedata packets for forwarding, the PDCP SDU to which the sequence numberis allocated. Details are not described.

If the first mapping relationship is different from the second mappingrelationship, the target base station sends, to the terminal on thefirst DRB, a data packet that includes no flow identifier in the datapackets received by the target base station via the tunnel establishedbased on a DRB, and the target base station sends, to the terminal onthe second DRB, a data packet that includes a flow identifier in thedata packets received by the target base station via the tunnelestablished based on a DRB. Specifically, when two tunnels areestablished between the target base station and the source base stationbased on a DRB, a data packet transmitted via a tunnel established basedon a third DRB of the source base station and the first DRB istransmitted on the first DRB, and a data packet transmitted via a tunnelestablished based on the DRB of the source base station and the secondDRB is transmitted on the second DRB. When only one tunnel isestablished between the target base station and the source base stationbased on the third DRB of the source base station and the first DRB, thetarget base station may route, to the second DRB via an SDAP entity, adata packet in the received data packets that includes a flowidentifier, and transmits the data packet on the second DRB. In thisimplementation, another data packet that includes no flow identifier issent on the first DRB. In this implementation, the source base stationmay obtain a flow identifier of a data packet for forwarding in a mannerin which an SDAP entity sends the flow identifier to the PDCP layerduring a handover, and indicate, in the data packet for forwarding, thatthe flow identifier is used for the handover. Alternatively, the sourcebase station may generate, in another flow identifier obtaining mannerdescribed in the embodiments of this application, flow identifiers forall PDCP layer data packets that have no PDCP SN, and forward, to thetarget base station, the data packets that include the flow identifier.Details are not described.

In this implementation, alternatively, the source base station maygenerate, in the various flow identifier obtaining manners described inthis application, a flow identifier for a PDCP layer data packet towhich no sequence number is allocated, and send, to the target basestation, the data packet for forwarding that includes the flowidentifier. Details are not described.

Optionally, if the forwarded data packets are PDCP layer data packets towhich no sequence number is allocated, some of the data packets includeflow a identifier, and a remaining data packet includes no flowidentifier, the forwarded data packets are sent to the terminal on thefirst DRB. This can improve data packet transmission continuity andaccuracy. A data packet of a non-access stratum flow that features inreflective QoS may be sent on the first DRB. The reflective QoS meansthat the flow features in uplink and downlink symmetry. To be specific,a flow in an uplink direction and a flow in a downlink direction havesame QoS, and an uplink traffic flow template (TFT) and a downlinktraffic flow template are symmetric. For example, an uplink sourceaddress and source port number are a downlink destination address anddestination port number, and an uplink destination address anddestination port number are a downlink source address and source portnumber. In this scenario, the access network device adds a flowidentifier to an air-interface data packet, and the terminal obtains,based on the received flow identifier and 5-tuple information of thedownlink data packet, QoS of a flow in an uplink direction and acorresponding TFT.

In the uplink direction, the terminal may send, to the target basestation on the first DRB or the second DRB, uplink data packets thatinclude the forwarded data packets.

Optionally, the uplink data packets sent by the terminal to the targetbase station include data packets that need continuous uplinktransmission, such as a PDCP SDU that fails to be transmitted by theterminal on the DRB of the source base station and to which a sequencenumber is allocated, and a PDCP SDU that fails to be transmitted by theterminal on the DRB of the source base station and to which no sequencenumber is allocated. All the uplink data packets include flowidentifiers.

Optionally, the target base station receives the uplink data packetsfrom the first DRB. Specifically, if the first mapping relationship isthe same as the second mapping relationship, the terminal may continueto send a PDCP layer data packet on the first DRB. The PDCP layer datapacket includes the unsuccessfully transmitted PDCP SDU to which thesequence number is allocated, and/or the unsuccessfully transmitted PDCPSDU to which no sequence number is allocated.

Optionally, the source base station sends a status report to theterminal, and the terminal determines a to-be-sent data packet based onthe status report. The status report may be a PDCP status report thatindicates a receiving status of a PDCP SDU on the reflect DRB of thetarget base station and that is used to notify the terminal to send aPDCP SDU that is incorrectly received on a receiving side. If the targetbase station does not send the PDCP status report, the terminal may sendall cached PDCP SDUs on the reflect DRB.

If the first mapping relationship is different from the second mappingrelationship, the terminal may continue to send, on the first DRB, alldata packets such as a PDCP SDU that is cached at a PDCP layer and towhich a sequence number is allocated, and/or a PDCP SDU that is cachedat the PDCP layer and to which no sequence number is allocated. Afterthe terminal completes sending of the PDCP layer data packets, an SDAPentity may route, to the second DRB for sending, another data packetsuch as a data packet cached at an SDAP layer. A PDCP entity of theterminal may notify the SDAP entity of the target base station that datapacket sending is completed.

Optionally, the target base station receives the uplink data packetsfrom the second DRB. The receiving manner is applicable to a scenario inwhich the first mapping relationship is different from the secondmapping relationship. Specifically, the terminal may sequentially send,to an SDAP entity, PDCP layer data packets corresponding to DRBs,including the unsuccessfully transmitted PDCP SDU to which a sequencenumber is allocated and/or the unsuccessfully transmitted PDCP SDU towhich no sequence number is allocated. The sequential sending meanssending the data packets to an SDAP layer in a sequence of receiving thedata packets by a PDCP layer from the SDAP layer. The SDAP entity isestablished based on a session, and all the data packets sent to theSDAP entity include a flow identifier. The PDCP PDUs include a datapacket that has been successfully received by the target base station.The PDCP entity of the terminal corresponding to the DRB of the targetbase station performs an operation such as sequence number removal ordecryption on the PDCP PDUs, to convert the PDCP PDUs into PDCP SDUs.Alternatively, the SDAP entity that is of the terminal and that iscorresponding to the DRB of the target base station removes sequencenumbers of the PDCP PDUs. The SDAP entity of the terminal routes, to acorresponding DRB (namely, the second DRB) based on the second mappingrelationship, the data packets received from the PDCP layer. The SDAPentity first sends, to the second DRB, the data packets received fromthe PDCP layer, and then sends, to the second DRB, a data packetreceived from an upper layer. It may be understood that in thisscenario, the target base station and the second access network devicemay not perform a data forwarding process in the uplink direction.

Optionally, the target base station receives, from the first DRB, a PDCPlayer data packet to which a sequence number is allocated in the uplinkdata packets, and receives, from the second DRB, a PDCP layer datapacket to which no sequence number is allocated in the uplink datapackets and/or an SDAP layer data packet in the uplink data packets. Thereceiving manner is applicable to a scenario in which the first mappingrelationship is different from the second mapping relationship.Specifically, the terminal may continue to send, on the first DRB, theunsuccessfully transmitted PDCP SDU to which the sequence number isallocated. The source base station may send a status report to theterminal, and the terminal determines a to-be-sent data packet based onthe status report. Details are not described. In addition, the terminalmay sequentially send the PDCP SDUs to an SDAP entity. All the datapackets sent to the SDAP entity include flow identifiers. The SDAPentity routes, to a corresponding DRB based on the second mappingrelationship, the data packets received from a PDCP layer. The SDAPentity first sends, to the corresponding DRB, the data packets receivedfrom the PDCP layer, and then sends, to the corresponding DRB, a datapacket received from an upper layer.

In an implementation of this application, when an unacknowledged mode(UM) service such as a cell broadcast service or an IP call service isperformed over an air interface, information about an air-interfacetransmission status of a data packet related to the service does notneed to be transferred between the target base station and the sourcebase station. Specifically, in the uplink direction, the source basestation sends a successfully received data packet to the core network,without performing data forwarding. The terminal accesses the targetbase station, and the terminal transmits a data packet on the new DRBbased on the second mapping relationship configured by the target basestation. This scenario is applicable to a scenario in which the firstmapping relationship is the same as or different from the second mappingrelationship. In the downlink direction, the source base stationforwards, to the target base station, a data packet has not beentransmitted and a new data packet received from the core network.Compared with an acknowledged mode (AM) service, in the unacknowledgedmode service, a data packet for forwarding does not include a PDCP SDUthat is on the DRB of the source base station and to which a sequencenumber is allocated. In other words, the source base station does notneed to send, to the target base station, a PDCP SDU unsuccessfully senton the DRB of the source base station. Other behavior of the target basestation or the source base station is consistent with that in a downlinkdata transmission process in an AM mode. Details are not described.

Optionally, after receiving the uplink data packets, the target basestation first delivers, in ascending order of PDCP SNs to the corenetwork, PDCP SDUs that are in the uplink data packets and to whichsequence numbers are allocated, and then delivers, to the core network,the PDCP SDU that is in the uplink data packets and to which no sequencenumber is allocated.

In an implementation of this application, in a process of transmittingthe data packets in the downlink direction or the uplink direction, datapackets that belong to a same flow may be transmitted on different DRBs.For example, some data packets in the same flow are transmitted on thereflect DRB, and a remaining data packet is transmitted on the new DRB.The reflect DRB and the new DRB are distinguished only in a timedimension. For example, a data packet first transmitted to the targetbase station is transmitted on the reflect DRB, and a data packet latertransmitted to the target base station is transmitted on the new DRB. Inthis case, the data packets in the same flow can be sequentiallytransmitted in either of the following manners.

Optionally, a transmit end controls a data packet transmission sequence.Specifically, after sending of forwarded data packets on the first DRBis completed, the second DRB is instructed to send another data packet.Optionally, a PDCP entity corresponding to the first DRB may notify aPDCP entity corresponding to the second DRB. If the first DRB includesdata packets of a plurality of flows, the first DRB may notify to asecond DRB to which each flow maps. Alternatively, after sending offorwarded data packets on the first DRB is completed, an SDAP entity isnotified. For example, a PDCP entity of the first DRB may notify thecorresponding SDAP entity that the sending of the forwarded data packetsis completed. If the PDCP entity corresponding to the first DRB canlearn a flow identifier, the PDCP entity may notify the correspondingSDAP entity that sending of data packets that are in the forwarded datapackets and that belong to one flow is completed. If the SDAP entitylearns that the forwarded data packets sent by the PDCP entity aresuccessfully sent, the SDAP entity starts to route a data packet of acorresponding flow to the corresponding second DRB, and the data packetof the corresponding flow is an SDAP layer data packet in a flow whosesending is completed on the first DRB.

Optionally, a receive end controls a data packet transmission sequence.Specifically, the receive end receives data packets in a same flow fromthe first DRB and the second DRB, and the receive end may distinguishbetween the data packets from the different DRBs based on an end marker,to sort the data packets received from the different DRBs. For example,the receive end first delivers, to an upper-layer protocol layer entity,a data packet that is of the flow and that is received from the firstDRB, and then delivers, to the upper-layer protocol layer entity, a datapacket that is of the flow and that is received from the second DRB. Theend marker is used to indicate that data packet transmission on thefirst DRB ends. The end marker may be an independent data packet orcontrol packet, for example, an independent SDAP layer or PDCP layerdata packet or control packet. Alternatively, the end marker mayindicate that a specific data packet such as a PDCP PDU is an endmarker.

It may be understood that in the downlink direction, the transmit end isa network side, for example, the target base station, and the receiveend is a terminal side; and in the uplink direction, the transmit end isa terminal side, and the receive end is a network side, for example, thetarget base station.

In an implementation of this application, if the PDCP layer of thesource base station obtains no flow identifier of the data packets, thedata packets for forwarding sent by the source base station to thetarget base station include no flow identifier. In this scenario, in thedownlink direction, the target base station may send the data packets tothe terminal in any one of the following manners:

When the data packets for forwarding are sent to the target base stationvia one or more tunnels established based on a DRB, the target basestation may send the forwarded data packets to the terminal on the firstDRB (namely, the reflect DRB), and then, after completing sending of theforwarded data packets, send, by still using the first DRB, another datapacket received from the core network via the SDAP layer. It may beunderstood that the sending method is applicable to either a scenario inwhich the first mapping relationship is the same as the second mappingrelationship or a scenario in which the first mapping relationship isdifferent from the second mapping relationship.

When the data packets for forwarding are sent to the target base stationvia one DRB-based tunnel and one tunnel established based on a session,the tunnel established based on a session may be used to transmit thedata packet cached at the SDAP layer, and the tunnel established basedon a DRB may be used to transmit the data packet cached at the PDCPlayer. Specifically, if the second mapping relationship is the same asthe first mapping relationship, the target base station may send theforwarded data packets to the terminal on the first DRB (reflect DRB),and then, after completing sending of the forwarded data packets, send,by still using the first DRB, another data packet received from the corenetwork via the SDAP layer. If the second mapping relationship isdifferent from the first mapping relationship, all forwarded datapackets on the DRB-based tunnel are sent on the corresponding reflectDRB of the target base station. The SDAP entity of the target basestation may route, to the new DRB, all forwarded data packets receivedfrom the tunnel established based on a session, and send the datapackets on the new DRB.

Optionally, the source base station may allocate a sequence number to aPDCP SDU cached at the PDCP layer, and send, to the target base stationas some of the data packets for forwarding, the PDCP SDU to which thesequence number is allocated. Details are not described.

When the data packets for forwarding are sent to the target base stationvia one tunnel established based on a session, the target base stationroutes the forwarded data packets to the second DRB based on the secondmapping relationship, and sends the data packets. The receive enddiscards all out-of-order data packets. Further, the receive end maynotify the target base station of an initial sequence number of adiscarded data packet or a sequence number of a last data packetdelivered to the core network. In this scenario, the target base stationmay not establish the reflect DRB. The forwarded data packets are alldata packets after an identified PDCP layer data packet, and theidentified data packet is used to indicate that the transmit end nolonger sends a data packet before the data packet, but the transmit endrepeatedly sends the data packets after the data packet.

According to the data transmission method provided in this embodiment ofthis application, in the downlink direction, the data packets forforwarding are transmitted via different types of tunnels, and theforwarded data packets are sent to the terminal in transmission mannerscorresponding to the tunnel types. In the uplink direction, the terminalsends the uplink data packets to the network side on different DRBs. Inthis embodiment of this application, various flexible data packettransmission manners are provided, and a data packet transmission mannercan be selected based on an actual network status, to avoid thefollowing problem: In a handover or dual connectivity or anotherscenario, a data packet is lost or is repeatedly transmitted becauseeach base station independently configures a mapping relationshipbetween a flow and a DRB. This improves terminal service continuity andimproves communication quality.

FIG. 7 is a schematic structural diagram of an access network device 70)according to an embodiment of this application.

The access network device 700 is applicable to the communications systemshown in FIG. 1. The access network device 700 may perform theoperations performed by the first access network device in theembodiment shown in FIG. 2 or FIG. 5 or the target base station in theembodiment shown in FIG. 6.

As shown in FIG. 7, the access network device 700 includes a receivingunit 701 and a processing unit 702.

The receiving unit 701 is configured to receive forwarded data packets(or a forwarded data packet) from a second access network device.

Optionally, in a handover scenario, the access network device 700 is atarget base station, and the second access network device is a sourcebase station.

Optionally, in a dual connectivity scenario, the access network device700 is a secondary base station, and the second access network device isa master base station; or the access network device 700 is a master basestation, and the second access network device is a secondary basestation.

The processing unit 702 is configured to map, to a first DRB, at leastone data packet in the forwarded data packets that includes a flowidentifier. The first DRB is corresponding to a DRB of the second accessnetwork device.

The first DRB and the DRB of the second access network device each meeta first mapping relationship, and the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device.

Optionally, the processing unit 702 is further configured to map, to thefirst DRB, a data packet in the forwarded data packets that includes noflow identifier.

Optionally, the processing unit 702 is further configured to map, to asecond DRB based on a second mapping relationship, at least one datapacket in the forwarded data packets that includes a flow identifier andthat is other than the data packet mapped to the first DRB. The secondmapping relationship is a mapping relationship between a flow and a DRBin the first access network device.

The first DRB is a reflect DRB that can maintain a transmission statusof the DRB of the second access network device. The second DRB is a newDRB that is established by the access network device 700 based on themapping relationship that is between a flow and a DRB and that isconfigured by the access network device 700, namely, the second mappingrelationship.

It may be understood that the first DRB may be used to transmit a datapacket that includes a flow identifier or a data packet that includes noflow identifier, and the second DRB may be used to transmit a datapacket that includes a flow identifier.

For detailed descriptions of the first DRB, the second DRB, the firstmapping relationship, and the second mapping relationship, refer to therelated content in another embodiment of this application. Details arenot described again.

Optionally, the processing unit 702 is further configured to release thefirst DRB after data packet sending on the first DRB is completed. Fordetailed descriptions, refer to the related content in anotherembodiment of this application. Details are not described again.

Optionally, in an implementation of this application, the receiving unit701 is specifically configured to receive the forwarded data packetsfrom the second access network device via a tunnel established based ona DRB and a tunnel established based on a session. The tunnelestablished based on a DRB may be used to transmit a PDCP layer datapacket that is of the second access network device and to which asequence number is allocated. The tunnel established based on a sessionmay be used to transmit an SDAP layer data packet of the second accessnetwork device, and/or the tunnel established based on a session is usedto transmit a PDCP layer data packet that is of the second accessnetwork device, that includes a flow identifier, and to which nosequence number is allocated. Then, the processing unit 702 may map, tothe first DRB, a data packet received via the tunnel established basedon a DRB, and map, to the second DRB, a data packet received via thetunnel established based on a session.

Optionally, in an implementation of this application, the receiving unit701 is specifically configured to receive the forwarded data packetsfrom the second access network device via one or more tunnelsestablished based on a DRB. Then, the processing unit 702 may map, tothe first DRB, the data packet in the received forwarded data packetsthat includes no flow identifier, and map, to the second DRB, the datapacket in the forwarded data packets that includes a flow identifier.

Optionally, in an implementation of this application, the receiving unit701 is specifically configured to receive, from the second accessnetwork device via a tunnel established based on a session, the datapacket in the forwarded data packets that includes a flow identifier.Then, the processing unit 702 may route, to the first DRB via an SDAPentity, a PDCP layer data packet that is in the forwarded data packetsand to which a sequence number is allocated, and route, to the secondDRB, a PDCP layer data packet to which no sequence number is allocatedor the SDAP layer data packet.

Optionally, the access network device 700 further includes a sendingunit 703, configured to send, to a terminal on the first DRB or on thefirst DRB and the second DRB, downlink data packets that include theforwarded data packets. For example, the sending unit 703 is configuredto: send, to the terminal on the first DRB, the PDCP layer data packetthat is in the forwarded data packets and to which a sequence number isallocated, and send, to the terminal on the second DRB, the PDCP layerdata packet that is in the forwarded data packets and to which nosequence number is allocated.

For detailed descriptions of receiving the forwarded data packets viadifferent types of tunnels and detailed descriptions of downlink datatransmission manners corresponding to the different types of tunnels,refer to the related content in another embodiment of this application.Details are not described herein again.

FIG. 8 is a schematic structural diagram of an access network device 800according to an embodiment of this application.

The access network device 800 is applicable to the communications systemshown in FIG. 1. The access network device 800 may perform theoperations performed by the first access network device in theembodiment shown in FIG. 3 or FIG. 5 or the target base station in theembodiment shown in FIG. 6.

The access network device 800 includes:

a receiving unit 801, configured to receive forwarded data packets (or aforwarded data packet) from a second access network device; and

a processing unit 802, configured to: map, to a first data radio bearerDRB, a data packet in the forwarded data packets that includes no flowidentifier, where the first DRB is corresponding to a DRB of the secondaccess network device: and map, to a second DRB based on a secondmapping relationship, a data packet in the forwarded data packets thatincludes a flow identifier, where the second mapping relationship is amapping relationship between a flow and a DRB in the first accessnetwork device.

The first DRB and the DRB of the second access network device each meeta first mapping relationship, and the first mapping relationship is amapping relationship between a flow and a DRB in the second accessnetwork device.

For detailed descriptions of the first DRB, the second DRB, the firstmapping relationship, and the second mapping relationship, refer to therelated content in another embodiment of this application. Details arenot described again.

Optionally, the processing unit 802 is further configured to route, viaan SDAP entity to the second DRB, the data packet in the forwarded datapackets that includes a flow identifier.

Optionally, in an implementation of this application, the access networkdevice 800 further includes a sending unit 803, configured to: send, toa terminal on the first DRB, a data packet in the forwarded data packetsthat includes no flow identifier and to which a sequence number isallocated: and send, to the terminal on the second DRB, a data packet inthe forwarded data packets that includes a flow identifier and to whichno sequence number is allocated.

Optionally, the processing unit 802 is further configured to release thefirst DRB after data packet sending on the first DRB is completed. Fordetailed descriptions, refer to the related content in anotherembodiment of this application. Details are not described again.

Optionally, in an implementation of this application, the receiving unit801 receives the forwarded data packets via a tunnel between the accessnetwork device 800 and the second access network device. In a downlinkdirection, the tunnel between the first access network device and thesecond access network device may be established in different manners.For detailed descriptions of receiving the forwarded data packets viadifferent types of tunnels and detailed descriptions of downlink datatransmission manners corresponding to the different types of tunnels,refer to the related content in another embodiment of this application.Details are not described herein again.

FIG. 9 is a schematic structural diagram of an access network device 90according to an embodiment of this application.

The access network device 900 is applicable to the communications systemshown in FIG. 1. The access network device 900 may perform theoperations performed by the second access network device in any one ofthe embodiments shown in FIG. 2 to FIG. 5 or the source base station inthe embodiment shown in FIG. 6.

The access network device 900 includes:

a processing unit 901, configured to generate a data packet forforwarding that includes a flow identifier (or data packets forforwarding that include flow identifiers); and

a sending unit 902, configured to send, to a first access networkdevice, the data packet for forwarding that includes a flow identifier(or data packets for forwarding that include flow identifiers).

Optionally, in a downlink direction, the data packet for forwardingincludes at least one type of the following data packets: a PDCP layerdata packet that is of the second access network device, to which asequence number is allocated, and for which no reception acknowledgementis obtained from a terminal; a PDCP layer data packet that is of thesecond access network device and to which no sequence number isallocated; and a Service Data Adaptation Protocol SDAP layer data packetof the second access network device.

Optionally, in the downlink direction, the sending unit 902 may befurther configured to send, to the first access network device, a datapacket for forwarding that includes no flow identifier.

Optionally, in the downlink direction, the sending unit 902 may beconfigured to send the data packet for forwarding to the first accessnetwork device via different types of tunnels. For detailed descriptionsof various tunnels and transmission of the data packet for forwarding inthe various tunnels, refer to the related content in another embodimentof this application. Details are not described herein again.

Optionally, in an implementation of this application, in the downlinkdirection, the processing unit may be further configured to obtain aflow identifier via a PDCP entity, to add the flow identifier to thedata packet for forwarding, and generate the data packet for forwardingthat includes the flow identifier, and then the first access networkdevice parses the forwarded data packet to obtain the flow identifier.For detailed descriptions of how to generate the data packet thatincludes a flow identifier, refer to the related content in theembodiment shown in FIG. 5. Details are not described herein again.

Optionally, in an uplink direction, the data packet for forwardingincludes an out-of-order data packet received from the terminal. Theaccess network device 900 may receive the out-of-order data packet byusing a receiving unit 903. All data packets for forwarding in theuplink direction include flow identifiers. The sending unit 902 may sendthe data packets for forwarding in the uplink direction to the firstaccess network device via a tunnel established based on a DRB. Detailsare not described herein.

FIG. 10 is a schematic structural diagram of an access network device1000 according to an embodiment of this application.

The access network device 1000 is applicable to the communicationssystem shown in FIG. 1. The access network device 1000 may perform theoperations performed by the first access network device in theembodiment shown in FIG. 4 or the operations performed by the targetbase station in the embodiment shown in FIG. 6.

The access network device 1000 includes:

a receiving unit 1001, configured to receive a forwarded data packet (orforwarded data packets) from a second access network device, where theforwarded data packet includes a flow identifier, and the forwarded datapacket includes an out-of-order data packet received by the secondaccess network device from a terminal; and

a sending unit 1002, configured to send the received forwarded datapacket to a core network device.

Optionally, the receiving unit 1001 receives the forwarded data packetvia a tunnel between the access network device 1000 and the secondaccess network device. The tunnel may be a tunnel that is establishedbetween the first access network device and the second access networkdevice based on a DRB. For the tunnel, refer to the related descriptionsof the tunnel established based on a DRB in another embodiment of thisapplication. Details are not described again.

The receiving unit 1001 is further configured to receive uplink datapackets from the terminal. The uplink data packets include at least onetype of the following data packets: a PDCP layer data packet that isunsuccessfully sent by the terminal to the second access network deviceand to which a sequence number is allocated: a PDCP layer data packetthat is of the terminal and to which no sequence number is allocated;and an SDAP layer data packet of the terminal.

Optionally, the receiving unit 1001 is specifically configured toreceive the uplink data packets on a first DRB.

Optionally, the receiving unit 1001 is specifically configured toreceive the uplink data packets on a second DRB.

Optionally, the receiving unit 1001 is specifically configured to:receive, on a first DRB, a PDCP layer data packet that is of theterminal and to which a sequence number is allocated in the uplink datapackets: and receive, on a second DRB, the PDCP layer data packet thatis of the terminal and to which no sequence number is allocated in theuplink data packets and/or the SDAP layer data packet of the terminal inthe uplink data packets.

Optionally, the receiving unit 1001 is specifically configured to:receive, on a first DRB, a PDCP layer data packet of the terminal in theuplink data packets; and receive, on a second DRB, the SDAP layer datapacket of the terminal in the uplink data packets.

The first DRB is a reflect DRB. The second DRB is a new DRB. Fordetailed descriptions of the first DRB and the second DRB, refer to therelated content in another embodiment of this application. Details arenot described again.

FIG. 11 is a schematic structural diagram of a terminal 1100 accordingto an embodiment of this application.

The terminal 1100 is applicable to the communications system shown inFIG. 1. The terminal 1100 may perform the operations performed by theterminal in any one of the embodiments shown in FIG. 2 to FIG. 6.

The terminal 1100 includes: a sending unit 1101, configured to senduplink data packets to an access network device, where the uplink datapackets include flow identifiers; and/or a receiving unit 1102,configured to receive downlink data packets from the access networkdevice, where at least one of the downlink data packets includes a flowidentifier.

The downlink data packets include a forwarded data packet (or forwardeddata packets) in a downlink direction. For specific content of theforwarded data packet, refer to the related descriptions in anotherembodiment of this application. Details are not described herein again.

Optionally, the sending unit 1101 is specifically configured to send theuplink data packets to the access network device on a first DRB.

Optionally, the sending unit 1101 is specifically configured to send theuplink data packets to the access network device on a second DRB.

Optionally, the sending unit 1101 is specifically configured to: send,to the access network device on a first DRB, a PDCP layer data packetthat is in the uplink data packets and to which a sequence number isallocated; and send, to the access network device on a second DRB, aPDCP layer data packet to which no sequence number is allocated in theuplink data packets and/or an SDAP layer data packet in the uplink datapackets. The PDCP layer data packet transmitted on the first DRB may bea data packet that includes a flow identifier, or may be a data packetthat includes no flow identifier.

Optionally, the sending unit 1101 is specifically configured to: send aPDCP layer data packet in the uplink data packets to the access networkdevice on a first DRB; and send an SDAP layer data packet in the uplinkdata packets to the access network device on a second DRB. The PDCPlayer data packet includes a data packet to which a sequence number isallocated and a data packet to which no sequence number is allocated.

Optionally, the receiving unit 1102 is specifically configured toreceive the downlink data packets from the access network device on thefirst DRB.

Optionally, the receiving unit 1102 is specifically configured toreceive, from the access network device on the second DRB, a downlinkdata packet that includes a flow identifier.

Optionally, the receiving unit 1102 is specifically configured to:receive, from the access network device on the first DRB, a data packetin the downlink data packets that includes no flow identifier; andreceive, from the access network device on the second DRB, a data packetin the downlink data packets that includes a flow identifier.

Optionally, the receiving unit 1102 is specifically configured to:receive, from the access network device on the first DRB, a PDCP layerdata packet that is in the downlink data packets and to which a sequencenumber is allocated; and receive, from the access network device on thesecond DRB, a PDCP layer data packet that includes a flow identifier andto which no sequence number is allocated in the downlink data packetsand an SDAP layer data packet in the downlink data packets.

The first DRB is a reflect DRB. The second DRB is a new DRB. Fordetailed descriptions of the first DRB and the second DRB, refer to therelated content in another embodiment of this application. Details arenot described again.

The access network device is an access network device that has beenaccessed by the terminal. When the access network device is an accessnetwork device accessed by the terminal after a handover or is an accessnetwork device that receives offloaded data in a dual connectivityprocess, the downlink data packets include a forwarded data packet in adownlink direction and another downlink data packet received by theaccess network device from a core network. For detailed descriptions,refer to the related content in another embodiment of this application.Details are not described again.

FIG. 12 is a schematic structural diagram of an access network device1200 according to an embodiment of this application.

The access network device 1200 is applicable to the communicationssystem shown in FIG. 1, the access network device 1200 may perform theoperations performed by the first access network device in any one ofthe embodiments shown in FIG. 2 to FIG. 5 or the target base station inthe embodiment shown in FIG. 6.

The access network device 1200 includes one or more remote radio units(RRU) 1201 and one or more baseband units (BBU) 1202. The RRU 1201 maybe referred to as a transceiver unit, a transceiver, a transceivercircuit, a transceiver, or the like, and the RRU 1201 may include atleast one antenna 12011 and a radio frequency unit 12012. The RRU 1201is mainly configured to: transmit/receive a radio frequency signal, andperform conversion between the radio frequency signal and a basebandsignal, for example, configured to send, to UE, information such as thesignaling indication described in the foregoing method embodiments. TheBBU 1202 is mainly configured to: perform baseband processing, controlthe access network device, and so on. The RRU 1201 and the BBU 1202 maybe physically disposed together; or may be physically disposedseparately, in other words, the access network device 1200 is adistributed access network device. When the access network deviceincludes a CU and a DU, a function of the RRU may be implemented by theDU, and a function of the BBU may be implemented by the CU; or afunction of the RRU and some functions of the BBU are implemented by theDU, and another function of the BBU is implemented by the CU; or somefunctions of the RRU are implemented by the DU, and another function ofthe RRU and a function of the BBU are implemented by the CU. This is notlimited.

The BBU 1202 is a control center of the access network device, may alsobe referred to as a processing unit, and is mainly configured tocomplete a baseband processing function, such as channel coding,multiplexing, modulation, or spread spectrum. For example, the BBU maybe configured to control the access network device 1200 to perform theoperations performed by the first access network device in any one ofthe embodiments shown in FIG. 2 to FIG. 5 or the target base station inthe embodiment shown in FIG. 6.

For example, the BBU 1202 may include one or more boards. A plurality ofboards may jointly support a radio access network (for example, an NRaccess network) of a single access standard, or may support radio accessnetworks of different access standards. The BBU 1202 further includes amemory 12021 and a processor 12022. The memory 12021 is configured tostore necessary instructions and necessary data. For example, the memory12021 stores a UE context in the foregoing embodiments. The processor12022 is configured to control the access network device 1200 to performnecessary actions, for example, configured to control the access networkdevice 1200 to perform the actions of the first access network device inany one of the embodiments shown in FIG. 2 to FIG. 4, or control theaccess network device 1200 to perform the actions of the target basestation in the embodiment shown in FIG. 5. The memory 12021 and theprocessor 12022 may serve the one or more boards. In other words, amemory and a processor may be separately disposed on each board, or aplurality of boards may share a same memory and processor. In addition,a necessary circuit is further disposed on each board.

For example, the BBU 1202 further includes a communications unit 12023.The communications unit 12023 is configured to support the accessnetwork device 1200 in communicating with a network element such asanother access network device or a core network device, for example,support the access network device 1200 in receiving a forwarded datapacket from a second access network device. The communications unit12023 may include a communications interface, for example, acommunications interface between the access network device 1200 and thesecond access network device, or a communications interface between theaccess network device 1200 and the core network device.

FIG. 13 is a schematic structural diagram of an access network device1300 according to an embodiment of this application.

The access network device 1300 is applicable to the communicationssystem shown in FIG. 1. The access network device 1300 may perform theoperations performed by the second access network device in any one ofthe embodiments shown in FIG. 2 to FIG. 5 or the source base station inthe embodiment shown in FIG. 6.

The access network device 1300 includes one or more RRUs 1301 and one ormore BBUs 1302.

The BBU 1302 may be configured to control the access network device 1300to perform the operations performed by the second access network devicein the embodiments shown in FIG. 2 to FIG. 5, or control the accessnetwork device 1300 to perform the operations performed by the sourceaccess network device in the embodiment shown in FIG. 6.

For example, the BBU 1302 may include one or more boards. A plurality ofboards may jointly support a radio access network (for example, an NRaccess network) of a single access standard, or may support radio accessnetworks of different access standards. The BBU 1302 further includes amemory 13021 and a processor 13022. The memory 13021 is configured tostore necessary instructions and necessary data. For example, the memory13021 stores a UE context obtained from the first access network devicein the foregoing embodiments. The processor 13022 is configured tocontrol the access network device 1300 to perform necessary actions, forexample, configured to control the access network device 1300 to performthe actions of the second access network device in the embodiments shownin FIG. 2 to FIG. 5, or control the access network device 1300 toperform the actions of the source base station in the embodiment shownin FIG. 6.

For example, the BBU 1302 further includes a communications unit 13023.The communications unit 13023 is configured to support the accessnetwork device 1300 in communicating with a network element such asanother access network device or a core network device, for example,support the access network device 1300 in sending a data packet forforwarding to a first access network device. The communications unit13023 may include a communications interface, for example, acommunications interface between the access network device 1300 and thefirst access network device, or a communications interface between theaccess network device 1300 and the core network device.

For detailed descriptions of functions of the RRU and the BBU anddetailed descriptions of functions of apparatuses such as the memory andthe processor in the BBU, refer to the related content in the embodimentshown in FIG. 13. Details are not described herein again.

FIG. 14 is a schematic structural diagram of a terminal 1400 accordingto an embodiment of this application.

The terminal 1400 is applicable to the communications system shown inFIG. 1. The terminal 1400 may perform the operations performed by theterminal in any one of the embodiments shown in FIG. 2 to FIG. 6.

For ease of description, FIG. 14 shows only main components of theterminal. As shown in FIG. 14, the terminal 1400 includes a processor, amemory, a control circuit, an antenna, and an input/output apparatus.The processor is mainly configured to: process a communications protocoland communication data, control entire user equipment, execute asoftware program, and process data of the software program, for example,configured to support the terminal 1400 in performing the terminalactions described in FIG. 2 to FIG. 6. The memory is mainly configuredto store the software program and the data, for example, store aterminal context described in the foregoing embodiments. The controlcircuit is mainly configured to: perform conversion between a basebandsignal and a radio frequency signal, and process the radio frequencysignal. The control circuit and the antenna may also be jointly referredto as a transceiver, mainly configured to transmit/receive a radiofrequency signal in a form of an electromagnetic wave, for example, maybe configured to: send an uplink data packet to an access networkdevice, or receive a downlink data packet from an access network device.For details, refer to the related descriptions in the methodembodiments. The input/output apparatus such as a touchscreen, a displayscreen, or a keyboard is mainly configured to: receive data entered by auser, and output data to the user.

After the terminal is powered on, the processor may read the softwareprogram in the storage unit, interpret and execute instructions of thesoftware program, and process the data of the software program. When theprocessor needs to send data wirelessly, the processor outputs abaseband signal to a radio frequency circuit after performing basebandprocessing on the to-be-sent data. After performing radio frequencyprocessing on the baseband signal, the radio frequency circuit sends aradio frequency signal in an electromagnetic wave form via the antenna.When data is sent to the terminal, the radio frequency circuit receivesa radio frequency signal via the antenna, converts the radio frequencysignal into a baseband signal, and outputs the baseband signal to theprocessor. The processor converts the baseband signal into data, andprocesses the data.

A person skilled in the art may understand that for ease of description,FIG. 14 shows only one memory and only one processor. Actually, theterminal may have a plurality of processors and a plurality of memories.The memory may also be referred to as a storage medium, a storagedevice, or the like. This is not limited in this embodiment of thisapplication.

In an optional implementation, the processor may include a basebandprocessor and a central processing unit. The baseband processor ismainly configured to process the communications protocol and thecommunication data. The central processing unit is mainly configured to:control the entire terminal, execute the software program, and processthe data of the software program. The processor in FIG. 14 integratesfunctions of the baseband processor and the central processing unit. Aperson skilled in the art may understand that the baseband processor andthe central processing unit may alternatively be separate processors,and are interconnected via a technology such as a bus. A person skilledin the art may understand that the terminal may include a plurality ofbaseband processors to adapt to different network standards, and theterminal may include a plurality of central processing units to enhancea processing capability of the terminal. The components in the terminalmay be connected via various buses. The baseband processor may also beexpressed as a baseband processing circuit or a baseband processingchip. The central processing unit may also be expressed as a centralprocessing circuit or a central processing chip. A function ofprocessing the communications protocol and the communication data may bebuilt in the processor, or may be stored in the storage unit in a formof a software program. The processor executes the software program toimplement a baseband processing function.

FIG. 15 is a schematic diagram of a communications system 1500 accordingto an embodiment of this application.

The communications system 1500 includes a first access network device1501 and a second access network device 1502.

The first access network device 1501 may perform the operationsperformed by the first access network device in any one of theembodiments shown in FIG. 2 to FIG. 5, or perform the operationsperformed by the target base station in the embodiment shown in FIG. 6.For example, the first access network device may be the access networkdevice in the embodiment shown in FIG. 7, FIG. 8, FIG. 10, or FIG. 12.

The second access network device 1502 may perform the operationsperformed by the second access network device in any one of theembodiments shown in FIG. 2 to FIG. 5, or perform the operationsperformed by the source base station in the embodiment shown in FIG. 6.For example, the second access network device may be the access networkdevice in the embodiment shown in FIG. 9 or FIG. 13.

The communications system may further include a terminal 1503 thatseparately communicates with the first access network device 1501 andthe second access network device 1502. The terminal 1503 may perform theoperations performed by the terminal in any one of the embodiments shownin FIG. 2 to FIG. 6, and may be the terminal in the embodiment shown inFIG. 11 or FIG. 14.

A person skilled in the art may clearly understand that mutual referencemay be made between the descriptions of the embodiments provided in thisapplication. For example, for the purpose of convenient and briefdescription, for functions and execution steps of each apparatus anddevice provided in the embodiments of this application, refer to therelated descriptions in the method embodiments of this application.Mutual reference may also be made between the method embodiments andbetween the apparatus embodiments.

All or some of the foregoing embodiments may be implemented by software,hardware, firmware, or any combination thereof. When being implementedby software, all or some of the foregoing embodiments may be implementedin a form of a computer program product. The computer program productincludes one or more computer instructions. When the computer programinstructions are loaded and executed on a computer, all or some of theprocedures or functions according to the embodiments of the presentinvention are generated. The computer may be a general-purpose computer,a dedicated computer, a computer network, or another programmableapparatus. The computer instructions may be stored in a computerreadable storage medium or may be transmitted from a computer readablestorage medium to another computer readable storage medium. For example,the computer instructions may be transmitted from a website, computer,server, or data center to another website, computer, server, or datacenter in a wired (for example, a coaxial cable, an optical fiber, or adigital subscriber line (DSL)) or wireless (for example, infrared,radio, or microwave) manner. The computer readable storage medium may beany usable medium accessible to a computer, or a data storage deviceintegrating one or more usable media, for example, a server or a datacenter. The usable medium may be a magnetic medium (for example, afloppy disk, a hard disk, or a magnetic tape), an optical medium (forexample, a DVD), a semiconductor medium (for example, a solid state disk(SSD)), or the like.

In the several embodiments provided in this application, it should beunderstood that the disclosed system, device, and method may beimplemented in other manners without departing from the scope of thisapplication. For example, the described embodiments are merely examples.For example, the module or unit division is merely logical functiondivision and may be other division during actual implementation. Forexample, a plurality of units or components may be combined orintegrated into another system, or some features may be ignored or notperformed. The units described as separate parts may or may not bephysically separate, and parts displayed as units may or may not bephysical units, in other words, may be located in one position, or maybe distributed on a plurality of network units. Some or all of themodules may be selected based on an actual requirement to achieve theobjectives of the solutions in the embodiments. A person of ordinaryskill in the art may understand and implement the embodiments of thisapplication without creative efforts.

In addition, the described system, device, method, and schematicdiagrams of different embodiments can be combined or integrated withanother system, module, technology, or method without departing from thescope of this application. In addition, the displayed or discussedmutual couplings or direct couplings or communication connections may beimplemented via some interfaces. The indirect couplings or communicationconnections between the apparatuses or units may be implemented inelectronic, mechanical, or other forms.

The foregoing descriptions are merely specific implementations of thisapplication, but are not intended to limit the protection scope of thisapplication. Any variation or replacement readily figured out by aperson skilled in the art within the technical scope disclosed in thisapplication shall fall within the protection scope of this application.Therefore, the protection scope of this application shall be subject tothe protection scope of the claims.

1. A data transmission method, comprising: establishing, by a secondaccess network device, a data radio bearer (DRB)-based tunnel and asession-based tunnel with a first access network device; sending, by thesecond access network device, one or more Packet Data ConvergenceProtocol (PDCP) layer data packets to the first access network devicevia the DRB-based tunnel; and sending, by the second access networkdevice, one or more Service Data Adaptation Protocol (SDAP) layer datapackets to the first access network device via the session-based tunnel.2. The method according to claim 1, wherein the one or more PDCP layerdata packets comprises one or more PDCP SDUs.
 3. The method according toclaim 1, wherein a first DRB of the first access network device, towhich the one or more PDCP layer data packets are mapped, iscorresponding to a DRB of the second access network device.
 4. Themethod according to claim 3, wherein a mapping relationship between oneor more flows and the DRB of the second access network device is thesame as a mapping relationship between the one or more flows and thefirst DRB.
 5. The method according to claim 3, wherein the first DRB andthe DRB of the second access network device have a same PDCP sequencenumber SN status and a same hyper frame number HFN status.
 6. The methodaccording to claim 4, wherein the method further comprises: sending, bythe second access network device, the mapping relationship between theone or more flows and the DRB of the second access network device to thefirst access network device.
 7. A communication apparatus, comprising atleast one processor coupled with a non-transitory storage medium storinginstructions that, when executed by the at least one processor, cause anaccess network device to: establish a data radio bearer DRB-based tunneland a session-based tunnel with a first access network device; and sendone or more Packet Data Convergence Protocol (PDCP) layer data packetsto the first access network device via the DRB-based tunnel; and sendone or more Service Data Adaptation Protocol (SDAP) layer data packetsto the first access network device via the session-based tunnel.
 8. Thecommunication apparatus according to claim 7, wherein the one or morePDCP layer data packets comprises one or more PDCP SDUs.
 9. Thecommunication apparatus according to claim 7, wherein a first DRB of thefirst access network device, to which the one or more PDCP layer datapackets are mapped, is corresponding to a DRB of a second access networkdevice.
 10. The communication apparatus according to claim 9, wherein amapping relationship between one or more flows and the DRB of the secondaccess network device is the same as a mapping relationship between theone or more flows and the first DRB.
 11. The communication apparatusaccording to claim 9, wherein the first DRB and the DRB of the secondaccess network device have a same PDCP sequence number SN status and asame hyper frame number HFN status.
 12. The communication apparatusaccording to claim 10, wherein the instructions when executed by the atleast one processor, further cause the access network device to: sendthe mapping relationship between the one or more flows and the DRB ofthe second access network device to the first access network device. 13.A communication system, comprising a first access network device and asecond access network device; wherein: the first access network deviceis configured to establishes a data radio bearer (DRB)-based tunnel anda protocol data unit (PDU) session-based tunnel with the second accessnetwork device; the second access network device is configured toforward one or more Packet Data Convergence Protocol (PDCP) layer datapackets to the first access network device via the DRB-based tunnel andforward one or more Service Data Adaptation Protocol (SDAP) layer datapackets to the first access network device via the session-based tunnel;and the first access network device is further configured to receive theone or more Packet Data Convergence Protocol (PDCP) layer data packetsfrom the second access network device via the DRB-based tunnel, andreceive the one or more Service Data Adaptation Protocol (SDAP) layerdata packets from the second access network device via the session-basedtunnel.
 14. The communication system according to claim 13, wherein theone or more PDCP layer data packets comprises one or more PDCP SDUs. 15.The communication system according to claim 13, wherein a first DRB ofthe first access network device, to which the one or more PDCP layerdata packets are mapped, is corresponding to a DRB of the second accessnetwork device.
 16. The communication system according to claim 15,wherein a mapping relationship between one or more flows and the DRB ofthe second access network device is the same as a mapping relationshipbetween the one or more flows and the first DRB.
 17. The communicationsystem according to claim 16, wherein the second access network deviceis configured to: send the mapping relationship between the one or moreflows and the DRB of the second access network device to the firstaccess network device; and the first access network device is configuredto: receive the mapping relationship between the one or more flows andthe DRB of the second access network device from the second accessnetwork device.
 18. The communication system according to claim 15,wherein the first access network device is configured to: after sendingthe one or more PDCP layer data packets to a terminal on the first DRB,send to the terminal on the second DRB, one or more data packetsreceived from a core network, which are of one or more flowscorresponding to the one or more PDCP layer data packets.
 19. Thecommunication system according to claim 15, wherein the first accessnetwork device is further configured to: receive data packets of one ormore flows from a terminal on the first DRB and a second DRB; and afterreceiving an end marker from the terminal on the first DRB, sending, bythe first access network device, one or more data packets of the one ormore flows received on the second DRB to a core network.
 20. Thecommunication system according to claim 19, wherein the first accessnetwork device is further configured to: send a mapping relationshipbetween the one or more flows and the second DRB to the terminal via thesecond access network device.